design: contracts: typo fix in example.

Change-Id: I73bce7bd3603a84cc539de0135fcefa4e45253ca
Reviewed-on: https://go-review.googlesource.com/132795
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/design/go2draft-contracts.md b/design/go2draft-contracts.md
index e4b714a..cdc0450 100644
--- a/design/go2draft-contracts.md
+++ b/design/go2draft-contracts.md
@@ -2033,7 +2033,7 @@
 // the receiver stops reading them.
 func Ranger(type T)() (*Sender(T), *Receiver(T)) {
 	c := make(chan T)
-	d := make(chan struct{})
+	d := make(chan bool)
 	s := &Sender(T){values: c, done: d}
 	r := &Receiver(T){values: c, done: d}
 	runtime.SetFinalizer(r, (*Receiver(T)).finalize)