Mark command line examples in Cronet iOS BUILD.md

Change-Id: I3f22338ab00c4f668b1d6dd4e263b6dddf9357da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625903
Reviewed-by: Zhongyi Shi <zhongyi@chromium.org>
Commit-Queue: Zhongyi Shi <zhongyi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663281}
1 file changed