aosp: fix broken link

Fix link to the Prerequisites page.

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 a951c84..e2a4c22 100644
--- a/building/aosp/aosp.rst
+++ b/building/aosp/aosp.rst
@@ -32,7 +32,7 @@
 	  optee-os.
 
 After following the AOSP setup instructions, the following additional packages
-from main `Prerequisites`_ page are needed. Please install them.
+from main :ref:`prerequisites` page are needed. Please install them.
 
 Build instructions
 ******************
@@ -175,4 +175,3 @@
           generic HiKey960 builds under P.
 
 .. _official instructions: https://source.android.com/source/devices.html
-.. _Prerequisites: https://optee.readthedocs.io/building/prerequisites.html#prerequisites