doc: fix a nit in fs_newalias.pod

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

Change-Id: I16ede184265e03a104fb724bece7fc461ca10415
Reviewed-on: http://gerrit.openafs.org/10595
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>