Demote sorted search to level 3 heading

[ellnix]

Just something I noticed while editing README.md: ## Sorted search seems to logically belong under ## Custom search but it has the same level of heading (and does not have its place in the TOC).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.22%. Comparing base (b5983bd) to head (7916b0c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #343 +/- ## ======================================= Coverage 90.22% 90.22% ======================================= Files 13 13 Lines 757 757 ======================================= Hits 683 683 Misses 74 74 ```

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[brunoocasali]

bors merge

[meili-bors[bot]]

Build succeeded:

• integration-tests (Rails 6.1 with Ruby 2.7)
• integration-tests (Rails 6.1 with Ruby 3.0)
• integration-tests (Rails 6.1 with Ruby 3.1)
• integration-tests (Rails 7 with Ruby 2.7)
• integration-tests (Rails 7 with Ruby 3.0)
• integration-tests (Rails 7 with Ruby 3.1)
• linter-check
• smoke-test