Corrects order of identifier and type in arguments list in 7.3.3 Constraints example. Fixes #1613

doc/rust.md

@@ -2750,7 +2750,7 @@ A _constraint_ is a predicate applied to specific slots.
 For example, consider the following code:
 
 ~~~~~~~~
-pure fn is_less_than(int a, int b) -> bool {
+pure fn is_less_than(a: int, b: int) -> bool {
      ret a < b;
 }