ubuntu version updated

Author: cl117

.github/workflows/slate-action.yml

@@ -2,7 +2,7 @@ on: push
 name: Create Slate Documentation on Push
 jobs:
   slate-documentation:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
     - name: Build documentation