Fixes hyperlink in customisation/index.md

[funkyfuture]

sorry, i haven't tested it as i conveniently followed the "edit page" button.

[pradyunsg]

Ah, thanks! While this specific change doesn't fix the issue, I've pushed https://github.com/pradyunsg/furo/commit/40b61f4789c9fae8492bb247ea585a8b8ce31071 for this! :)

[funkyfuture]

Thank you!