Don't use British English spelling

Summary: For consistency reasons use "Behavior", not "Behaviour" .

Reviewers: #plasma, #vdg, ngraham, rooty

Reviewed By: #vdg, ngraham, rooty

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10273

Differential Revision: https://phabricator.kde.org/D18899