Use the field name for the show hint link, like Anki desktop.

Fix for [issue 1788](https://code.google.com/p/ankidroid/issues/detail?id=1788), although that report asks that the “Show ” in “Show NN” should be remov

It’s called [`tag`](https://github.com/dae/anki/blob/3ee193731007ca8df28cbe9534218a70d97599fa/anki/template/hint.py#L17) in the Anki desktop code and somewhere in the `args` in AnkiDroid.

I checked all calls to `runFilter` and we always put in a fourth argument.

The original issue report asks that the “Show ” in “Show NN” should be removed. This brings it in line with Anki desktop instead, which has the “Show” too.

P.S.: I untabified line 41.