ci: Update apt before installing packages. (#28)

Author: jamesiarmes

.github/workflows/docs.yaml

@@ -22,7 +22,7 @@ jobs:
         with:
           python-version: 3.x
       - name: Install plantuml
-        run: sudo apt-get install -y plantuml
+        run: sudo apt-get update && sudo apt-get install -y plantuml
       - run: echo "cache_id=$(date --utc '+%V')" >> $GITHUB_ENV
       - uses: actions/cache@v4
         with: