hg: code-tools/trees: Fix minor typo:

[jonathan.gibbons at oracle.com]

Changeset: a789f1e84919
Author:    jjg
Date:      2017-06-08 13:31 -0700
URL:       http://hg.openjdk.java.net/code-tools/trees/rev/a789f1e84919

Fix minor typo:
Contributed-by: tim.bell at oracle.com

! trees.py