docs: add a name to Xcode url

This patch adds a name to the xcode url, so when browsing
https://chromium.googlesource.com/chromium/src/+/master/docs/mac_build_instructions.md
it will be nicer to read.

BUG=None
R=sdefresne@chromium.org

Review URL: https://codereview.chromium.org/1731783005

Cr-Commit-Position: refs/heads/master@{#377977}
1 file changed