Fix repo check-out in CI

Author: gsvgit

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - name: Set up Git repository
-      - uses: actions/checkout@v2
+        uses: actions/checkout@v2
 
       # First run of pdflatex.
       - name: pdflatex main