fix(shared-docs): extract captured group from heading custom id regexp

[lacolaco]

String#match doesn't return captured group for a "global" pattern. To get it, RegExp#exec() is appropriate. The previous test case was passed with a string that converted the entire header to an ID even though the capture failed, so it was fixed to match the custom id exactly.

This PR fixes my fault in #2195

[lacolaco]

@JeanMeche @josephperrott Sorry for the repeated bother. Could you please take a look at this?

[josephperrott]

This PR was merged into the repository by commit 722ce5b647ec89797633452dd293e1dbac776ff1.

The changes were merged into the following branches: main

[angular-automatic-lock-bot[bot]]

This issue has been automatically locked due to inactivity. Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}