Fix a typo in the test_treenode_stats unit test
Fix a typo in the test_treenode_stats unit test

htem/CATMAID,fzadow/CATMAID,fzadow/CATMAID,fzadow/CATMAID,htem/CATMAID,htem/CATMAID,fzadow/CATMAID,htem/CATMAID