ci: Use Pull Request Head Ref

.github/workflows/check-toc.yml

@@ -17,7 +17,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           token: ${{ secrets.GH_PAT || github.token }}
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.ref }}
 
       - name: Setup Node
         uses: actions/setup-node@v3

.github/workflows/check.yml

@@ -13,7 +13,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           token: ${{ secrets.GH_PAT || github.token }}
-          ref: ${{ github.head_ref }}
+          ref: ${{ github.event.pull_request.head.ref }}
 
       - name: Setup Node
         uses: actions/setup-node@v3

CONTRIBUTING.md

@@ -199,7 +199,7 @@ Publishing is handled by a 2-branch [pre-release process](https://intuit.github.
 - PRs made into the default branch are auto-deployed to the `next` pre-release tag on NPM. The result can be installed with `npm install vega-lite/@next`.
   - When merging into `next`, please use the `squash and merge` strategy.
 - To release a new stable version, open a PR from `next` into `stable` using this [compare link](https://github.com/vega/vega-lite/compare/stable...next).
-  - When merging from `next` into `stable`, please use the `create a merge commit` strategy. 
+  - When merging from `next` into `stable`, please use the `create a merge commit` strategy.
   - After the release note is generated (from commits), please take a look to clean up so the log is readable to the community.
 
 ## Suggested Programming Environment.