fix: update github actions environment

Author: ruanyf

.github/workflows/wangdoc.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   page-generator:
     name: Generating pages
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v2