Fix typos in docbook

Summary:
c6a767100086 and 2b1c8bec0fd1 forgot to add markup to make
mentions of "Spectacle" entities. By changing to &spectacle;, the
proper uppercase spelling is used in every case, too, which was not the
case before.

Test Plan:
Start Spectacle, press F1, check consistent spelling of
"Spectacle".

Reviewers: Spectacle, yurchor

Reviewed By: yurchor

Subscribers: kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D13860