Rename worker threads from xfsdump's documentation

While we've already removed the word 'slave' from the code, the
documentation should still be updated.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>