Update make_docs.yaml

bump cache to v4

Author: haeussma

.github/workflows/make_docs.yaml

@@ -17,7 +17,7 @@ jobs:
       - uses: actions/setup-python@v4
         with:
           python-version: 3.x
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         with:
           path: .cache
           key: ${{ github.ref }}