docs(customization.md): fix grammar mistake, add title to code blocks

[bearomorphism]

Description

Checklist

• I have read the contributing guidelines

Documentation Changes

• Run poetry doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external) in the documentation

When running poetry doc, any broken internal documentation links will be reported in the console output like this:

INFO    -  Doc file 'config.md' contains a link 'commands/bump.md#-post_bump_hooks', but the doc 'commands/bump.md' does not contain an anchor '#-post_bump_hooks'.

Expected Behavior

Steps to Test This Pull Request

Additional Context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.96%. Comparing base (120d514) to head (8f781ee).
Report is 747 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1535      +/-   ##
==========================================
+ Coverage   97.33%   97.96%   +0.62%     
==========================================
  Files          42       58      +16     
  Lines        2104     2703     +599     
==========================================
+ Hits         2048     2648     +600     
+ Misses         56       55       -1     

Flag	Coverage Δ
unittests	97.96% <ø> (+0.62%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.