Rename activities context menu item in task manager to "Show in Activities"

Summary: Sync with change in D23779.

Test Plan: Open the context menu for a task manager item.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23785