Fix typo in option hint

[mlasson]

Description

vsce package --help shows

Usage: vsce package|pack [options] [version]

Packages an extension

Options:
  -o, --out <path>                Output .vsix extension file to <path> location (defaults to <name>-<version>.vsix)
...
  --allow-missing-repository      Allow missing a repository URL in package.json
  --allow-unused-files-pattern    Allow include patterns for the files field in package.json that does not match any file
...

But the errors messages says:

The following include patterns in the "files" property in package.json do not match any files packaged in the extension:
  - LICENSE
Remove any include pattern which is not needed.

=> Run vsce ls --tree to see all included files.
=> Use --allow-unused-files-patterns to skip this check

This PR propoes to remove the "s" from the error message (because adding an s to the option would be a breaking change).