MAINTAINERS: The docs tree has moved

For years I've waited in vain for Linus to see the error of his ways and go
back to our beloved 2.6.x naming scheme.  But dispair has overtaken me at
last, so, with a heavy heart, I'm moving my repo to a more contemporary
name.  My kids will be proud of me.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>