Change reference to Mattermost versions.
Change reference to Mattermost versions.

Changed reference to Mattermost versions, so that it's clear which
menu navigation path applies.

mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq