win_cross.md: fix headings

TBR=hans@chromium.org

Bug: 
Change-Id: I6b5c5cf4b90abfc42d40fdc678ba2efcf2466913
Reviewed-on: https://chromium-review.googlesource.com/733873
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510891}
1 file changed