Fix typo in comment for gmtime macro defines
diff --git a/library/platform_util.c b/library/platform_util.c
index 2dd530d..89f132a 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -84,7 +84,7 @@
* preprocessor conditions above. Ideally, we could expose this macro in
* platform_utils.h and simply use it in platform_utils.c, threading.c and
* threading.h. However, this macro is not part of the Mbed TLS public API, so
- * we keep it private by only definining it in this file
+ * we keep it private by only defining it in this file
*/
#define PLATFORM_UTIL_USE_GMTIME
#endif /* !_POSIX_VERSION || _POSIX_C_SOURCE > _POSIX_THREAD_SAFE_FUNCTIONS */