Update step_07.ngdoc to fix typo in route pattern

[jamiekrug]

One instance of /phones/:phoneId erroneously had a singular version, /phone/:phoneId, which does not match what was actually used in the code.

[mary-poppins]

Thanks for the PR! Please check the items below to help us merge this faster. See the contributing docs for more information.

• Uses the issue template (#7313)

If you need to make changes to your pull request, you can update the commit with git commit --amend.
Then, update the pull request with git push -f.

Thanks again for your help!

[jamiekrug]

I started to fill out the issue template for this, but the "Login to Update Issue" authorization requests read and write access to all public and private repositories, which makes me very uneasy. This is a single-letter typo fix to documentation. I'd like to help out and follow helpful work flows, but that issue template authorization seems way overreaching, no? Thanks!

[petebacondarwin]

No problem! Thanks for the fix. We don't need all this for a simple docs fix.