Update default branch in workflow steps

Author: LKedward

.github/workflows/CI.yml

@@ -80,7 +80,7 @@ jobs:
       run: |
         mkdir -p previews/${{env.BUILD_DIR}}
         cp tex/paper.pdf previews/${{env.BUILD_DIR}}/
-        test -f tex/master.pdf && cp tex/master.pdf previews/${{env.BUILD_DIR}}/paper-diffmaster.pdf || true
+        test -f tex/main.pdf && cp tex/main.pdf previews/${{env.BUILD_DIR}}/paper-diffmain.pdf || true
 
     - name: Upload paper preview
       uses: EndBug/add-and-commit@v7
@@ -115,7 +115,7 @@ jobs:
         issue-number: ${{github.event.number}}
         body: |
           Preview this branch rendered at <${{env.REPO_URL}}/blob/previews/${{env.BUILD_DIR}}/paper.pdf>
-          Preview the latexdiff rendered at <${{env.REPO_URL}}/blob/previews/${{env.BUILD_DIR}}/paper-diffmaster.pdf>
+          Preview the latexdiff rendered at <${{env.REPO_URL}}/blob/previews/${{env.BUILD_DIR}}/paper-diffmain.pdf>
           <details>
           <summary>Click to view TexCount output</summary>