Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/psa/crypto_compat.h b/include/psa/crypto_compat.h
index 1d3253c..89c13f1 100644
--- a/include/psa/crypto_compat.h
+++ b/include/psa/crypto_compat.h
@@ -44,7 +44,7 @@
 
 #define PSA_KEY_HANDLE_INIT MBEDTLS_SVC_KEY_ID_INIT
 
-/** Check whether an handle is null.
+/** Check whether a handle is null.
  *
  * \param handle  Handle
  *