Cleaned up the docstring for readline.set_completer().

Thanks to Nathaniel Gray for reporting the confusion.

Modules/readline.c

@@ -266,7 +266,7 @@ static char doc_set_completer[] = "\
 set_completer([function]) -> None\n\
 Set or remove the completer function.\n\
 The function is called as function(text, state),\n\
-for i in [0, 1, 2, ...] until it returns a non-string.\n\
+for state in 0, 1, 2, ..., until it returns a non-string.\n\
 It should return the next possible completion starting with 'text'.\
 ";