Fix typos in docbook

Summary:
c6a76710 and 2b1c8bec 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 {key 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