doc: Update Command Line Tools download procedure

The instruction on downloading Command Line Tools through XCode has been
changed. This patch updates the building documentation to reflect the
changes.

Author: jeenalee

BUILDING.md

@@ -20,8 +20,9 @@ Prerequisites:
 
 On OS X, you will also need:
 * [Xcode](https://developer.apple.com/xcode/download/)
-  * You also need to install the `Command Line Tools` via Xcode. You can find 
-    this under the menu `Xcode -> Preferences -> Downloads`
+  * You also need to install the `Command Line Tools` via Xcode. You can find
+    this under the menu `Xcode -> Open Developer Tool -> More Developer
+    Tools...`
   * This step will install `gcc` and the related toolchain containing `make`
 
 On FreeBSD and OpenBSD, you may also need: