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}

docs/mac_build_instructions.md

@@ -5,7 +5,7 @@
 ## Prerequisites
 
 *   A Mac running 10.9+.
-*   https://developer.apple.com/xcode, 5+
+*   [Xcode](https://developer.apple.com/xcode), 5+.
 *   Install
     [gclient](http://dev.chromium.org/developers/how-tos/install-depot-tools),
     part of the