Update PULL_REQUEST_TEMPLATE.md (pytorch#41812)

Summary:
**Summary**
This commit updates the repository's pull request template to remind contributors to tag the issue that their pull request addresses.

**Fixes**
This commit fixes pytorch#35319.

Pull Request resolved: pytorch#41812

Reviewed By: gmagogsfm

Differential Revision: D22667902

Pulled By: SplitInfinity

fbshipit-source-id: cda5ff7cbbbfeb89c589fd0dfd378bf73a59d77b

Author: Meghan Lele

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1 @@
+Fixes #{issue number}