fix typo in PULL_REQUEST_TEMPLATE.md

Author: dunglas

.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 exist, explain below instead -->
+| Tickets | Fix #... <!-- prefix each issue number with "Fix #", no need to create an issue if none exists, explain below instead -->
 | License | MIT
 | Doc PR | symfony/symfony-docs#... <!-- required for new features -->
 <!--