Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/xtea.c b/library/xtea.c
index 77f6cb6..01478ac 100644
--- a/library/xtea.c
+++ b/library/xtea.c
@@ -1,5 +1,5 @@
 /*
- *  An 32-bit implementation of the XTEA algorithm
+ *  A 32-bit implementation of the XTEA algorithm
  *
  *  Copyright The Mbed TLS Contributors
  *  SPDX-License-Identifier: Apache-2.0