fix function name in tabnanny documentation (GH-764)

diff --git a/Lib/tabnanny.py b/Lib/tabnanny.py
index 76665ac..e48b72a 100755
--- a/Lib/tabnanny.py
+++ b/Lib/tabnanny.py
@@ -59,7 +59,7 @@
 
 class NannyNag(Exception):
     """
-    Raised by tokeneater() if detecting an ambiguous indent.
+    Raised by process_tokens() if detecting an ambiguous indent.
     Captured and handled in check().
     """
     def __init__(self, lineno, msg, line):