Fix typo and broken link in TypeAdapter description in "Why" docs

[ADSteele916]

Change Summary

I changed the misspelled NampedTuple to NamedTuple and updated its hyperlink, which appears to have been accidentally broken by #7417.

Related issue number

The PR template says that an issue is only required for non-trivial changes. If need be, I can create one for this PR to track.

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: @samuelcolvin

[codspeed-hq]

CodSpeed Performance Report

Merging #8703 will not alter performance

_{Comparing ADSteele916:fix-why-docs-typeadapter (8b51aa8) with main (8898b8f)}

Summary

✅ 10 untouched benchmarks

[ADSteele916]

My apologies; I don't seem to have permission to add a documentation label. Could a maintainer please review when possible?

[sydney-runkle]

Amazing, thanks!!