MB-100: corrected typespec for ns_cluster:do_complete_join

Somehow R14's dialyzer suddenly started seeing that obvious issue.

Change-Id: I4f300a6ba92b5b5254f38fd7b7f881ca9da32a96
Reviewed-on: http://review.couchbase.org/29879
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>