[application-dashboard] Use appropriate search string

Summary: "Type to search." is changed to "Type to search..." because there is a user input action that needs to be performed.

Reviewers: ngraham, #plasma, #vdg

Reviewed By: ngraham, #vdg

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20073