Correct documentation for Marshal (#287)

include "yaml:" qualifier in Marshal docs.

* Update yaml.go

yaml.go

@@ -140,7 +140,7 @@ func unmarshal(in []byte, out interface{}, strict bool) (err error) {
 // For example:
 //
 //     type T struct {
-//         F int "a,omitempty"
+//         F int `yaml:"a,omitempty"`
 //         B int
 //     }
 //     yaml.Marshal(&T{B: 2}) // Returns "b: 2\n"