docs: Update issue template naming Issue Helper (#3171)

This mentions Issue Helper application explicitely as the required entry
point for issue submission.

Thanks!

.github/ISSUE_TEMPLATE.md

@@ -1,8 +1,8 @@
-IMPORTANT: Please use the following link to create a new issue:
+IMPORTANT: Please use our Issue Helper to create a new issue:
 
 https://new-issue.vuejs.org/?repo=vuejs/vue-cli
 
-If your issue was not created using the app above, it will be closed immediately.
+If your issue was not created using Issue Helper from the link above, it will be closed immediately.
 
 中文用户请注意：
 请使用上面的链接来创建新的 issue。如果不是用上述工具创建的 issue 会被自动关闭。