Fix a/an typos in doxygen and other comments

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 5099cfb..7d99720 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -756,7 +756,7 @@
  *                 for the associated \c mbedtls_ssl_get_timer_t callback to
  *                 return correct information.
  *
- * \note           If using a event-driven style of programming, an event must
+ * \note           If using an event-driven style of programming, an event must
  *                 be generated when the final delay is passed. The event must
  *                 cause a call to \c mbedtls_ssl_handshake() with the proper
  *                 SSL context to be scheduled. Care must be taken to ensure