docs: fix typo in check_boxes (#4499)

Typo in documentation
check_boxex > check_boxes

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www/docs/includes/repository.md

@@ -40,7 +40,7 @@
         #
         # Since: v1.20 (pro).
         # This feature is only available in GoReleaser Pro.
-        check_boxex: true
+        check_boxes: true
 
         # Base can also be another repository, in which case the owner and name
         # above will be used as HEAD, allowing cross-repository pull requests.