Fix choice of language used in algorithm

Author: benjie

spec/Section 2 -- Language.md

@@ -1253,7 +1253,7 @@ Type : Name
 
 - Let {name} be the string value of {Name}.
 - Let {type} be the type defined in the Schema named {name}.
-- {type} must not be {null}.
+- {type} must exist.
 - Return {type}.
 
 Type : [ Type ]