Meta: escape space in issue link template URL

Author: nschonni

index.bs

@@ -6599,7 +6599,7 @@ It would not be appropriate to accept such a union, only to then convert values
 [=numeric type=] to a {{bigint}} for further processing, as this runs the risk of introducing
 precision errors.
 Please
-<a href="https://github.com/heycam/webidl/issues/new?title=Intent%20to%20use%20bigint/numeric union">file an issue</a>
+<a href="https://github.com/heycam/webidl/issues/new?title=Intent%20to%20use%20bigint/numeric%20union">file an issue</a>
 before using this feature.
 </p>