Make "Restart to update" message clearer

In https://blog.torproject.org/comment/288256#comment-288256 we received some feedback that the Restart to update Tor Browser message is not clear about what should be restarted. This comes from Firefox's message choice (updateRestart.panelUI.label2).

I'm not sure what we should do right now, as Mozilla may not be interested in changing their phrasing (but maybe they will).

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