Change sample workflow to use heredoc to more safely dump markdown to…

[jayqi]

Updates the sample GitHub Actions workflow to improve the "Write output to file" step. The previous version of this step breaks if the markdown content contains an apostrophe—see #25.

The proposed version uses heredoc to write out the file. Single quotes are used to avoid any variable substitution or backtick evaluation.

Here's a demo of it working successfully:

• GitHub Actions workflow run
• Input markdown
• Created release