Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index 211ea8a..a71ca3f 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -545,7 +545,7 @@
  * \retval #PSA_ERROR_INVALID_HANDLE
  *         \p key is not a valid identifier nor \c 0.
  * \retval #PSA_ERROR_COMMUNICATION_FAILURE
- *         There was an failure in communication with the cryptoprocessor.
+ *         There was a failure in communication with the cryptoprocessor.
  *         The key material may still be present in the cryptoprocessor.
  * \retval #PSA_ERROR_DATA_INVALID
  *         This error is typically a result of either storage corruption on a