Change Run Chromium section in mac_build_instructions.md

Bug: 
Change-Id: I0e7d08124fa62d3da99b76783de21c9dce06dcc4
Reviewed-on: https://chromium-review.googlesource.com/552904
Reviewed-by: Sidney San Martin <[email protected]>
Commit-Queue: Owen Min <[email protected]>
Cr-Commit-Position: refs/heads/master@{#483144}
1 file changed