Fix variable name in api/v<n>/snapshot.py

Noticed in a recent bug fix that the when the show
method was copy/pasted at some point the variable name
vol was not updated.

Silly annoyance, but thought I'd go ahead and replace vol
with snapshot to make the variable name reflect what's
actually in use here.

Change-Id: I137ecc4987258450e93195cfed5e128063b92914
Merge "Fix variable name in api/v<n>/snapshot.py"