Fix copy/pasta in label_short_desc

[chadwhitacre]

Why:

I'm working on adding label names to our GitHub events pipeline so that I can derive some metrics from label changes. Based on prior art in our repo I'm pulling payload examples from the docs to use as test fixture. This is where I ran across label_short_desc in the wild, and I was confused because it made me think maybe the labels payload is about the application of a label to an issue, when in fact of course it's about the label itself.

What's being changed:

The word "issue" is being changed to "label" (with subordinate emendation in preservation of articular agreement), fixing presumably a scribal error deriving from the derivation of label_short_desc from issue_short_desc without sufficient original modification of the original, herein forsooth forthwith corrected.

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

@chadwhitacre Thanks so much for opening a PR! I'll get this triaged for review ⚡

[ethanpalm]

@chadwhitacre This looks great 👍 Thanks for catching that typo and opening a PR to fix it! I'll update the branch and get this merged posthaste once tests are passing. 🍏