Fix return value of match_indices in pattern example

[laurmaedje]

The pattern example on Regex assumes that std's match_indices returns a pair of offsets while it actually returns an offset and a matching string slice. This wasn't catched by tests because the example is ignored (it needs nightly).

[BurntSushi]

Thanks!

Although it's likely that I'll probably remove support for this, as I don't think the Pattern feature has any current path to stability.