doc: fix a nit in fs_newalias.pod

The CAUTIONS section is about fs newalias, not fs newcell.

Reviewed-on: http://gerrit.openafs.org/10595
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 8b8e8c2ab1974ba4c14942cc7bd94aac8602192f)

Change-Id: Ie800b600f85f17362d92ce51ee55de3f24a3361e
Reviewed-on: http://gerrit.openafs.org/10601
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>