D5053: Fix terminology: use Plasma for the desktop

[Marco Martin]

mart accepted this revision.

REPOSITORY
  R119 Plasma Desktop

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D5053

To: ltoscano, apol, mart
Cc: apol, plasma-devel, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas