[startkde] Rename method startKSMServer to reflect real behaviour

Summary:
Since ksmserver was split, start-plasma launches plasma-session which in
turn invokes ksmserver.

Test Plan: No code changes, only a method rename

Reviewers: #plasma, broulik, apol

Reviewed By: #plasma, broulik, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27621