checkout with submodule

Author: toshimaru

.github/workflows/ci.yml

@@ -17,6 +17,8 @@ jobs:
     name: Rails Doc Build
     steps:
     - uses: actions/checkout@v2
+      with:
+        submodules: true
     - uses: ruby/setup-ruby@v1
       with:
         ruby-version: 2.7