Fix "skip empty branch pipelines" example

What does this MR do?

As explained in #15170 (comment 1133365836), using * isn't good enough for most real-world use cases, as it only matches files in the top level directory. Using **/* instead will match everything recursively.

I also changed from refs/heads/main to just main to simplify the example, as the keyword supports the short notation as well.

Correct behavior verified in https://gitlab.com/gl-demo-ultimate-mgrabowski/i-15170

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