Improve markdown checklist example in README

The example did not escape the markdown document correctly. Use an
environment variable instead of direct string interpolation to consume
the document safely in the github-script action.

Closes #59

Author: elopez

README.md

@@ -361,11 +361,13 @@ jobs:
     - name: Create/update checklist as PR comment
       uses: actions/github-script@v6
       if: github.event_name == 'pull_request'
+      env:
+        REPORT: ${{ steps.slither.outputs.stdout }}
       with:
         script: |
           const script = require('.github/scripts/comment')
           const header = '# Slither report'
-          const body = `${{ steps.slither.outputs.stdout }}`
+          const body = process.env.REPORT
           await script({ github, context, header, body })
 ```