README: it's better to use release version in GH action

Author: ekalinin

README.md

@@ -359,7 +359,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - run: |
-          curl https://raw.githubusercontent.com/ekalinin/github-markdown-toc/master/gh-md-toc -o gh-md-toc
+          curl https://raw.githubusercontent.com/ekalinin/github-markdown-toc/0.8.0/gh-md-toc -o gh-md-toc
           chmod a+x gh-md-toc
           ./gh-md-toc --insert --no-backup foo.md
       - uses: stefanzweifel/git-auto-commit-action@v4