D8239: Use correct .desktop file name when setting KMail as default

[Daniel Vrátil]

dvratil added a reviewer: Plasma.

REPOSITORY
  R119 Plasma Desktop

REVISION DETAIL
  https://phabricator.kde.org/D8239

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