Documentation/development-process: use -next trees instead of staging

This is confusing, as we have "staging" trees for drivers/staging.  Call
them -next trees.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>