minor symfony#52189 Tickets vs issues (fabpot)

This PR was merged into the 5.4 branch.

Discussion
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Tickets vs issues

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | n/a
| License       | MIT

GitHub uses `Issues`, we're even using `issue` in the comment there. Let's be consistent.

Commits
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07f9d2e Replace Tickets by Issues

Author: fabpot

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,7 +4,7 @@
 | Bug fix?      | yes/no
 | New feature?  | yes/no <!-- please update src/**/CHANGELOG.md files -->
 | Deprecations? | yes/no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
-| Tickets       | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
+| Issues        | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
 | License       | MIT
 
 <!--