Update error message to reference python3.

Unversioned python at the command line no longer works for me
(and presumably on new machines in general).

Change-Id: I9e64d7bc7c0e66fb75a926098c7e5740f136d068
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/660559
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
1 file changed