Update PULL_REQUEST_TEMPLATE.md

By adding the relevant issue number, the issue will automatically be closed when the PR is merged.

Author: Kyaa-dost

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,5 @@
-Issue: #
-<!-- Link to relevant issue. All PRs should be associated with an issue -->
+Fixes #
+<!-- Link to relevant issue (for ex: #1234) which will automatically close the issue once the PR is merged -->
 
 ## PR Type
 What kind of change does this PR introduce?