Update c.yml

`As noted above from @ebrannin-bw, upgrading to use actions/checkout@v3 should correct the deprecations. If it doesn't, please re-open this with some more information about your workflow :)`
actions/checkout#1047

.github/workflows/c.yml

@@ -47,7 +47,7 @@ jobs:
     runs-on: macos-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - name: Install dependencies
         run: |
           brew update && brew install criterion readline lcov gcovr