Update headless gui version

Author: dstansby

.github/workflows/docs.yml

@@ -32,7 +32,7 @@ jobs:
           sudo apt install graphviz --yes
 
       - name: Build Docs
-        uses: aganders3/headless-gui@v1
+        uses: aganders3/headless-gui@v2
         with:
           run: make html
           working-directory: ./docs