gob: Doc typo fix

Fixes #1785.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4496042

src/pkg/gob/doc.go

@@ -159,7 +159,7 @@ description, constructed from these types:
 		Elem typeId
 		Len  int
 	}
-	type CommonType {
+	type CommonType struct {
 		Name string // the name of the struct type
 		Id  int    // the id of the type, repeated so it's inside the type
 	}