TAP 15: update reference implementation link (#160)

Link to the PR which was merged into the python-tuf re-implementation
implementing succinct hashed bin delegations. Add a comment to update
further once docs are available.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

tap15.md

@@ -276,4 +276,5 @@ does not, Bob will not be able to verify J.
 
 # Augmented Reference Implementation
 
-https://github.com/theupdateframework/python-tuf/pull/1106
+<!-- TODO: add link to API docs and example once available -->
+https://github.com/theupdateframework/python-tuf/pull/2010