Fix missing article and comma (docs)

[bittner]

This is a trivial change in the documentation.

Change Summary

Fixes two minor English grammar issues at the beginning of the "Why use Pydantic?" chapter.

Related issue number

n/a

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @alexmojaki

[bittner]

please review

[codspeed-hq]

CodSpeed Performance Report

Merging #9500 will not alter performance

_{Comparing bittner:patch-1 (15e57af) with main (87adc65)}

Summary

✅ 13 untouched benchmarks

[alexmojaki]

Thanks!