Fix npm run serve example in the docs' README

[jhrozek]

Summary

The previous example npm run serve -p 3001 was giving me errors.

Fixes #(related issue)

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

follow the steps in the readme

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 52.473% (-0.009%) from 52.482%
when pulling c186ac7 on jhrozek:misc
into fe18235 on stacklok:main.