Do not capitalize placeholder username

Summary:
The current placeholder text for the username is "John". Usernames can't be capitalized so change it to "john".

BUG: 408478
FIXED-IN: 5.17.0

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22104