[Core] Remove --add-plugin alternate name from USAGE.txt

[ebreck]

--add-plugin is no longer a valid synonym for --plugin; it was removed in 839dba7 . But the USAGE still listed it as a synonym.

[codecov]

Codecov Report

Merging #2319 (3ef079d) into main (b73b38b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #2319   +/-   ##
=========================================
  Coverage     83.36%   83.36%           
  Complexity     2320     2320           
=========================================
  Files           300      300           
  Lines          8272     8272           
  Branches        750      750           
=========================================
  Hits           6896     6896           
  Misses         1079     1079           
  Partials        297      297           

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[aslakhellesoy]

Hi @ebreck,

Thanks for your making your first contribution to Cucumber, and welcome to the Cucumber committers team! You can now push directly to this repo and all other repos under the cucumber organization! 🍾

In return for this generous offer we hope you will:

• ✅ Continue to use branches and pull requests. When someone on the core team approves a pull request (yours or someone else's), you're welcome to merge it yourself.
• 💚 Commit to setting a good example by following and upholding our code of conduct in your interactions with other collaborators and users.
• 💬 Join the community Slack channel to meet the rest of the team and make yourself at home.
• ℹ️ Don't feel obliged to help, just do what you can if you have the time and the energy.
• 🙋 Ask if you need anything. We're looking for feedback about how to make the project more welcoming, so please tell us!

On behalf of the Cucumber core team,
Aslak Hellesøy
Creator of Cucumber

[mpkorstanje]

Cheers!