design: actually invoke set.Make(int) in contracts example

Change-Id: Id8d50959fa7d05e9911821d9b252b0ac51e0e6f6
GitHub-Last-Rev: 760f58ad8d0114897e3ba94472ec038bbf2f3284
GitHub-Pull-Request: golang/proposal#22
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/210917
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/design/go2draft-contracts.md b/design/go2draft-contracts.md
index dc1e74a..bd00879 100644
--- a/design/go2draft-contracts.md
+++ b/design/go2draft-contracts.md
@@ -2323,7 +2323,7 @@
 Example use:
 
 ```Go
-	s := set.Make(int)
+	s := set.Make(int)()
 	s.Add(1)
 	if s.Contains(2) { panic("unexpected 2") }
 ```