Clarifications of AOSP build instructions based on personal experience setting up. Also, correction of a wrongly constructed sentence.

Change-Id: I993015a8318d36341749388bf936f668641b2082
diff --git a/src/source/downloading.jd b/src/source/downloading.jd
index 81a636f..76c24b2 100644
--- a/src/source/downloading.jd
+++ b/src/source/downloading.jd
@@ -100,7 +100,7 @@
 $ repo init -u https://android.googlesource.com/platform/manifest
 </pre>
     <p>
-      To check out a branch other than "master", specify it with -b:
+      To check out a branch other than "master", specify it with <code>-b</code>. For a list of branches, see <a href="https://source.android.com/source/build-numbers.html#source-code-tags-and-builds">Source Code Tags and Builds</a>.
     </p>
 <pre>
 $ repo init -u https://android.googlesource.com/platform/manifest -b android-4.0.1_r1