[SDDM theme] Use grammatically correct string "Log In"

Summary: "Login" is a noun: "I have a login for the system." This is not appropriate for a button title that is used as an action; the correct verb-based string is "Log In".

Test Plan:

Reviewers: VDG, Plasma, abetts

Reviewed By: VDG, abetts

Subscribers: abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D16035