Change ubuntu version in docs workflow (#237)

Author: ravinkohli

.github/workflows/docs.yml

@@ -3,7 +3,7 @@ on: [pull_request, push]
 
 jobs:
   build-and-deploy:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - name: Setup Python