Fix letter case and grammar in Routing guide

[djpowers]

This minor commit changes "concerns" to lowercase to match the heading, and removes the "s" from "allow" to reflect the pluralization.
[ci skip]

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @eileencodes (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[akshay-vishnoi]

Please add [ci skip] in commit message in future for doc changes. This skips running test cases. Thanks