Fix format-security error in rlgl_standalone.c (#1305)

See #1304

examples/others/rlgl_standalone.c

@@ -219,7 +219,7 @@ int main(void)
 // GLFW3: Error callback
 static void ErrorCallback(int error, const char *description)
 {
-    fprintf(stderr, description);
+    fprintf(stderr, "%s", description);
 }
 
 // GLFW3: Keyboard callback