docs: One last fix to the Snackbar docs

Had an uncommited change I forgot to upload when I merged
the new Snackbar class--d'oh!

Since this is a small fix, I'll verify it, +2 it myself, and merge.
See first comment for doc stage location.

bug: 25191776
Change-Id: I1593f81d09a2607a559f0341ff827c79c6e00a70