fix typo in SkTaskGroup.h

This was meant to stay -1 (num_cores() threads) not to become 1 (1 thread).

Change-Id: Ib9b8e1963c0cd16d4c0aba02ccf7a65890a5ef78
Reviewed-on: https://skia-review.googlesource.com/8851
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed