Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/asn1write.h b/include/mbedtls/asn1write.h
index 44afae0..5da7654 100644
--- a/include/mbedtls/asn1write.h
+++ b/include/mbedtls/asn1write.h
@@ -90,7 +90,7 @@
 
 #if defined(MBEDTLS_BIGNUM_C)
 /**
- * \brief           Write a arbitrary-precision number (#MBEDTLS_ASN1_INTEGER)
+ * \brief           Write an arbitrary-precision number (#MBEDTLS_ASN1_INTEGER)
  *                  in ASN.1 format.
  *
  * \note            This function works backwards in data buffer.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 1cd6eb6..1da9802 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1329,7 +1329,7 @@
  * Include backtrace information with each allocated block.
  *
  * Requires: MBEDTLS_MEMORY_BUFFER_ALLOC_C
- *           GLIBC-compatible backtrace() an backtrace_symbols() support
+ *           GLIBC-compatible backtrace() and backtrace_symbols() support
  *
  * Uncomment this macro to include backtrace information
  */
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index 84fafd2..9cea40a 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -215,7 +215,7 @@
 int mbedtls_md_setup( mbedtls_md_context_t *ctx, const mbedtls_md_info_t *md_info, int hmac );
 
 /**
- * \brief           This function clones the state of an message-digest
+ * \brief           This function clones the state of a message-digest
  *                  context.
  *
  * \note            You must call mbedtls_md_setup() on \c dst before calling
diff --git a/include/mbedtls/ripemd160.h b/include/mbedtls/ripemd160.h
index 63270d1..f890aef 100644
--- a/include/mbedtls/ripemd160.h
+++ b/include/mbedtls/ripemd160.h
@@ -74,7 +74,7 @@
 void mbedtls_ripemd160_free( mbedtls_ripemd160_context *ctx );
 
 /**
- * \brief          Clone (the state of) an RIPEMD-160 context
+ * \brief          Clone (the state of) a RIPEMD-160 context
  *
  * \param dst      The destination context
  * \param src      The context to be cloned
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 062df73..8559f67 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -491,7 +491,7 @@
  *             the current function does not have access to them,
  *             and therefore cannot check them. See mbedtls_rsa_complete().
  *             If you want to check the consistency of the entire
- *             content of an PKCS1-encoded RSA private key, for example, you
+ *             content of a PKCS1-encoded RSA private key, for example, you
  *             should use mbedtls_rsa_validate_params() before setting
  *             up the RSA context.
  *             Additionally, if the implementation performs empirical checks,
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 5064ec5..9cfb00c 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -652,7 +652,7 @@
  *                 for the associated \c mbedtls_ssl_get_timer_t callback to
  *                 return correct information.
  *
- * \note           If using a event-driven style of programming, an event must
+ * \note           If using an event-driven style of programming, an event must
  *                 be generated when the final delay is passed. The event must
  *                 cause a call to \c mbedtls_ssl_handshake() with the proper
  *                 SSL context to be scheduled. Care must be taken to ensure