"require" instead of "register" in the extension registration docs

Hello!

In this page it looks like "require" should be used instead of "register".

Source: https://docs.antora.org/antora/latest/extend/register-extension/

I discovered this after getting this error:

$ npx antora antora-playbook.yml 
[13:31:28.546] FATAL (antora): The "request" argument must be of type string. Received type undefined
Add the --stacktrace option to see the cause of the error.

Which doesn't really tell me what went wrong :-D but I found the "require" key in the other docs, and it works.