aosp: add optional tag info for git clone

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
diff --git a/building/aosp/aosp.rst b/building/aosp/aosp.rst
index cb4ac5c..9157895 100644
--- a/building/aosp/aosp.rst
+++ b/building/aosp/aosp.rst
@@ -39,7 +39,7 @@
 
 .. code-block:: bash
 
-    $ git clone https://github.com/linaro-swg/optee_android_manifest
+    $ git clone https://github.com/linaro-swg/optee_android_manifest [-b <release_tag>]
     $ cd optee_android_manifest