fix output header in bookmaker

change underscore to space if needed
remove bmh_ prefix

TBR=caryclark@google.com
Bug: skia:
Change-Id: I9d4d29c7ff91d9d29bf8740d163724f371e5e211
Reviewed-on: https://skia-review.googlesource.com/28044
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
2 files changed