Use title case for the clipboard button

Summary: Use Title Case for the Copy to Clipboard button. In this case, it means making the word "to" lowercase.

Test Plan: Compile, deploy, open Spectacle, look at button title and docbook

Reviewers: Spectacle, KDE Applications, broulik, cfeck, Localization, lueck

Subscribers: pino, yurchor, kde-doc-english

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D15083