Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/psa/crypto_values.h b/include/psa/crypto_values.h
index 8b3a815..8e61f2f 100644
--- a/include/psa/crypto_values.h
+++ b/include/psa/crypto_values.h
@@ -1195,7 +1195,7 @@
  */
 #define PSA_ALG_CHACHA20_POLY1305               ((psa_algorithm_t)0x05100500)
 
-/* In the encoding of a AEAD algorithm, the bits corresponding to
+/* In the encoding of an AEAD algorithm, the bits corresponding to
  * PSA_ALG_AEAD_TAG_LENGTH_MASK encode the length of the AEAD tag.
  * The constants for default lengths follow this encoding.
  */