Polish contributing guide wording

[GabrielDenardi]

What changed

• I corrected the contributing guide to say a common validator is composed of two files, matching the list that follows.
• I smoothed a few wording issues in the validator testing, documentation, and checks sections.

Why

I wanted to make the guide clearer for new contributors following the validator creation workflow.

Validation

I ran the project QA checks in a Linux Docker container using PHP 8.5.6, with the required gettext and bcmath extensions installed:

• php bin/console lint:spdx
• php vendor/bin/phpcs
• php vendor/bin/phpstan analyze --memory-limit=1G
• php vendor/bin/phpunit --testsuite=unit
• php vendor/bin/pest --testsuite=feature --compact
• php bin/console lint:docs
• php bin/console smoke-tests:check-complete

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.41%. Comparing base (7d833fb) to head (8fd78ba).
⚠️ Report is 9 commits behind head on 3.1.

Additional details and impacted files

@@            Coverage Diff            @@
##                3.1    #1776   +/-   ##
=========================================
  Coverage     99.41%   99.41%           
  Complexity     1020     1020           
=========================================
  Files           194      194           
  Lines          2387     2387           
=========================================
  Hits           2373     2373           
  Misses           14       14           

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[alganet]

Thanks for your contribution 🐼