fix(docs): add osx-universal flag to usage.txt

[erickzhao]

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Summarize your changes:

In #1346, we added support for the --arch=universal flag, but never properly documented CLI usage of the associated OsxUniversalOptions.

For the future, we should also document the @electron/universal options better. We currently have no docs around these flags outside of the source code.

I tested the syntax by running the following command:

DEBUG=electron-universal npx electron-packager . --arch=universal --osx-universal.mergeASARs="true"

From here, I was able to confirm that the Universal package was receiving the options properly:

  electron-universal making a universal app with options {
  mergeASARs: 'true',
  x64AppPath: '/var/folders/wy/zjrnw32x2496ydq6ps7vljfc0000gp/T/electron-packager/electron-packager-universal-sTH01I/electron-quick-start-darwin-x64/electron-quick-start.app',
  arm64AppPath: '/var/folders/wy/zjrnw32x2496ydq6ps7vljfc0000gp/T/electron-packager/electron-packager-universal-sTH01I/electron-quick-start-darwin-arm64/electron-quick-start.app',
  outAppPath: '/var/folders/wy/zjrnw32x2496ydq6ps7vljfc0000gp/T/electron-packager/darwin-universal/electron-quick-start-darwin-universal-ZMutzn/electron-quick-start.app'
}

[codecov]

Codecov Report

Merging #1533 (94ccb54) into main (72bc2a5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1533   +/-   ##
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  Coverage   95.58%   95.58%           
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  Files          15       15           
  Lines         793      793           
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  Hits          758      758           
  Misses         35       35           

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