Use title case for the clipboard button

Summary: Use Title Case for the {nav 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