Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index e43ef96..002551f 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -479,7 +479,7 @@
  *             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,