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