IssueTemplate: remove \n at checkbox line

ISSUE_TEMPLATE.md

@@ -17,5 +17,4 @@
 <!-- Optional but greatly speeds up debugging. You can drag and drop a zip archive to upload it. -->
 
 
-- [ ] I searched the existing [GitHub issues](https://github.com/godotengine/godot/issues?utf8=%E2%9C%93&q=is%3Aissue+)
-  for potential duplicates.
+- [ ] I searched the existing [GitHub issues](https://github.com/godotengine/godot/issues?utf8=%E2%9C%93&q=is%3Aissue+) for potential duplicates.