Replace 'thread safe' by 'thread-safe' in the documentation
diff --git a/include/mbedtls/platform_util.h b/include/mbedtls/platform_util.h
index 82b1fd0..66a8221 100644
--- a/include/mbedtls/platform_util.h
+++ b/include/mbedtls/platform_util.h
@@ -68,7 +68,7 @@
#if defined(MBEDTLS_HAVE_TIME_DATE)
/**
- * \brief Thread safe implementation of gmtime()
+ * \brief Thread-safe implementation of gmtime()
*
* The function is an abstraction that when called behaves similar
* to the gmtime() function from the C standard, but is thread