Fix `Capybara::SessionConfig` API doc
Fix `Capybara::SessionConfig` API doc

- Fix invalid `@param` usage in the `#test_id=` method doc.
  See <https://www.rubydoc.info/gems/capybara/3.21.0/Capybara/SessionConfig#test_id=-instance_method>.

[skip ci]

jnicklas/capybara,jnicklas/capybara,teamcapybara/capybara,teamcapybara/capybara,jnicklas/capybara,teamcapybara/capybara