Fix contribute link in PR template (https://github.com/bazelbuild/bazel/pull/30885)

### Description

Fixes 'contributing' link in PR template. The `.html` causes it to 404.

### Build API Changes

No

### Checklist

- [ ] ~I have added tests for the new use cases (if any).~
- [ ] ~I have updated the documentation (if applicable).~

### Release Notes

RELNOTES: None

Closes #30885.

PiperOrigin-RevId: 971907259
Change-Id: I47e85f59a6e75de1f2aa4e3fb13bb4640d59e865

.github/pull_request_template.md

diff --git a/.github/pull_request_template.md b/.github/pull_request_template.md
index 62ea9e0..fd8c727 100644
--- a/.github/pull_request_template.md
+++ b/.github/pull_request_template.md
@@ -1,6 +1,6 @@
 <!--
 Thank you for contributing to Bazel!
-Please read the contribution guidelines: https://bazel.build/contribute.html
+Please read the contribution guidelines: https://bazel.build/contribute
 -->
 
 ### Description