go2draft-contracts.md: fix typo in vertex example

FromTo.Nodes is a method declaration.

Change-Id: I45c90d3a4a8062766fbcd0050471587f46499608
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/188117
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/design/go2draft-contracts.md b/design/go2draft-contracts.md
index ff6b586..dfa20a8 100644
--- a/design/go2draft-contracts.md
+++ b/design/go2draft-contracts.md
@@ -551,7 +551,7 @@
 type Vertex struct { ... }
 func (v *Vertex) Edges() []*FromTo { ... }
 type FromTo struct { ... }
-type (ft *FromTo) Nodes() (*Vertex, *Vertex) { ... }
+func (ft *FromTo) Nodes() (*Vertex, *Vertex) { ... }
 ```
 
 There are no interface types here, but we can instantiate