Tools: Update get-contributor-list script to allow -beta

[sdixon194]

Changes proposed in this Pull Request:

When releasing a beta, the script currently won't let you check a version number in CHANGELOG.md such as 10.4-beta. This update will allow that match.

Jetpack product discussion

N/a

Does this pull request change what data or activity we track or use?

n/a

Testing instructions:

Easiest way is to go to Jetpack's CHANGELOG.md file and append -beta to the 10.4 release section, so it reads 10.4-beta.

Then try running tools/get-contributor-list.sh plugins/jetpack 10.4. Things should work fine. Without the extra regex, the script will fail.

[github-actions]

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[anomiex]

This looks like it will work.