Some URL links are broken on PULL_REQUEST_TEMPLATE

[yudoufu]

Fix some broken URLs on .github/PULL_REQUEST_TEMPLATE.

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• [ ] Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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