Fix typo in set-nth documentation

lib/sass/script/functions.rb

@@ -170,7 +170,7 @@ module Sass::Script
   # \{#nth nth($list, $n)}
   # : Returns a specific item in a list.
   #
-  # \{#length set-nth($list, $n, $value)}
+  # \{#set-nth set-nth($list, $n, $value)}
   # : Replaces the nth item in a list.
   #
   # \{#join join($list1, $list2, \[$separator\])}