Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/md.h b/include/mbedtls/md.h
index 612061d..fbf938d 100644
--- a/include/mbedtls/md.h
+++ b/include/mbedtls/md.h
@@ -199,7 +199,7 @@
 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