Update "Click to Copy" button label in circuit display

Apparently the circuit display has a button to click to copy the URL, that appears when you hover over it (which is news to me!):

I'm guessing this is a legacy feature carried across from torbutton, since it's trivial enough to copy the URL from the address bar itself? In any case, we should avoid "Click..." commands in our microcopy and update the label to something like Copy Address instead.