Fix internal link to "Theming"

[dzienisz]

No description provided.

[gchtr]

Hey @dzienisz

Thanks a lot for taking the time to make a pull request! Changes to the docs are actually made in the main Timber repository https://github.com/timber/timber/tree/master/docs. The file that you changed is auto-generated via Hugo and would be overwritten with the next release of the docs.

The problem with actually lies somewhere else (timber/timber#1676 (comment)) and I’ll see that this will be fixed.