Fix typo in viewer docs

No-Try: true
Docs-Preview: https://skia.org/?cl=151663
Bug: skia:
Change-Id: I7c535c6c13251210d9842f1ddbb7425dd222a423
Reviewed-on: https://skia-review.googlesource.com/151663
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
1 file changed