Update docs preview URL in presubmit

NoTry: true
Bug: skia:
Change-Id: Id5b374f707ac44770a5142f104202d6509410767
Reviewed-on: https://skia-review.googlesource.com/129505
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
1 file changed