docs: update README with correct tax country code and pricing methods

[christiandopico]

Changes

Fix compile errors in README example:

• Updates the README invoice example to use the correct tax and ISO country-code helpers, pointer-based pricing, and the current tax category constant.

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Added thorough tests with at least 90% code coverage.
• Modified or created example GOBL documents to show my changes in use, if appropriate.
• Added links to the source of the changes in tax regimes or addons, either structured or in the comments.
• Run go generate . to ensure that the Schemas and Regime data are up to date.
• Reviewed and fixed all linter warnings.
• Been obsessive with pointer nil checks to avoid panics.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

And if you are part of the org:

• Requested a review from Copilot and fixed or dismissed (with a reason) all the feedback raised.
• Requested a review from @samlown.

[samlown]

Thanks for this!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (8652979) to head (8ca6383).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #935   +/-   ##
=======================================
  Coverage   94.99%   94.99%           
=======================================
  Files         327      327           
  Lines       17732    17732           
=======================================
  Hits        16845    16845           
  Misses        547      547           
  Partials      340      340           

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