Change notification button text to "View Updates"

Summary:
This is more accurate; "Update" inaccurately implies that updates will be immediately
installed, hereas "View updates" correctly indicates that you will be able to see what
the updates are before deciding whether or not to install them.

Reviewers: #vdg, #discover_software_store, apol, ndavis

Reviewed By: #vdg, ndavis

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26889