Fix error message for not-equal when type checking (#768)

According to the standard https://github.com/dhall-lang/dhall-lang/blob/master/standard/dhall.abnf\#L399 the correct operator is `!=`

Author: basile-henry

dhall/src/Dhall/TypeCheck.hs

@@ -3488,7 +3488,7 @@ prettyTypeMessage (CantEQ expr0 expr1) =
         buildBooleanOperator "==" expr0 expr1
 
 prettyTypeMessage (CantNE expr0 expr1) =
-        buildBooleanOperator "/=" expr0 expr1
+        buildBooleanOperator "!=" expr0 expr1
 
 prettyTypeMessage (CantInterpolate expr0 expr1) = ErrorMessages {..}
   where