Update GH build file

Updating to v3

Author: SFEMark

.github/workflows/mkdocs.yml

@@ -28,7 +28,7 @@ jobs:
       # - run: pip install beautifulsoup4==4.9.3
 
       - name: Set up build cache
-        uses: actions/cache@v2
+        uses: actions/cache@v3
         with:
           key: ${{ github.ref }}
           path: .cache