Fixed a typo.

doc/library/scan.txt

@@ -133,8 +133,8 @@ This makes it safe to pass a very long arange, which we need to do for generalit
 arange must have its length specified at creation time.
 
 
-Simple accumulation into a scalar, ditching lamba
--------------------------------------------------
+Simple accumulation into a scalar, ditching lambda
+--------------------------------------------------
 
 Although this example would seem almost self-explanatory, it stresses a
 pitfall to be careful of: the initial output state that is supplied, that is