Wrong @return comment block in StackedRouteMatchInterface interface

Good catch

Thanks for the patch. In order clean up these sorts of coding standards fairly and efficiently, many instances of identical fixes that can be reviewed with git diff --color-words should be combined into a single patch, ideally one that fixes all instances in core in preparation for enabling automated coding standards checking for them. This issue is postponed for now on #2571965: [meta] Fix PHP coding standards in core, stage 1.

If the standard being fixed already has a coder rule, this issue should me marked as a duplicate of the issue that begins to fix that rule. If there is not yet a coder rule, we should add one. See for example #2606860: Docblock sections out of order in core/lib/Drupal.

Thanks for your ongoing efforts to clean up the codebase and fix these bugs!

@xjm pointed out this might not be catchable by coder, so..
Committed/pushed to 8.1.x and cherry-picked to 8.0.x. Thanks!