chore: add closing keyword to PR template (#32065)

Author: mdrxy

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,7 +14,7 @@ Thank you for contributing to LangChain! Follow these steps to mark your pull re
 
 - [ ] **PR message**: ***Delete this entire checklist*** and replace with
   - **Description:** a description of the change. Include a [closing keyword](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword) if applicable to a relevant issue.
-  - **Issue:** the issue # it fixes, if applicable
+  - **Issue:** the issue # it fixes, if applicable (e.g. Fixes #123)
   - **Dependencies:** any dependencies required for this change
   - **Twitter handle:** if your PR gets announced, and you'd like a mention, we'll gladly shout you out!