Corrected WARNING on ErrorCallback()

The default compiler flags on Ubuntu includes -Wformat -Wformat-security which is what gives this error. That flag is used as a precaution against introducing security related bugs.

src/core.c

@@ -529,8 +529,8 @@ bool IsGamepadButtonUp(int gamepad, int button)
 // GLFW3 Error Callback, runs on GLFW3 error
 static void ErrorCallback(int error, const char *description)
 {
-    printf(description);
-    //fprintf(stderr, description);
+    //printf(description);
+    fprintf(stderr, "%s", description);
 }
 
 // GLFW3 Keyboard Callback, runs on key pressed
@@ -688,4 +688,4 @@ static void TakeScreenshot()
     free(imgDataPixel);
     
     shotNum++;
-}
+}