Update PULL_REQUEST_TEMPLATE.md (#7827)
Update PULL_REQUEST_TEMPLATE.md (#7827)

Change `Implements` to `Closes` so GitHub will automagically close issues referenced as such in new PRs.
This will help new contributors resolve issues more seamlessly from their PRs.

See [relevant PR discussion](https://github.com/ampproject/amphtml/issues/4949#issuecomment-282834618)
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