revert changes in check-md-link.yml

`check-md-link` requires a Docker environment, while `ubuntu-slim`
already runs inside Docker, so reverting this change.

Author: zhjwpku

.github/workflows/check-md-link.yml

@@ -37,7 +37,7 @@ permissions:
 
 jobs:
   markdown-link-check:
-    runs-on: ubuntu-slim
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
     - uses: tcort/github-action-markdown-link-check@e7c7a18363c842693fadde5d41a3bd3573a7a225