protogen: update Options documentation. protogen.Run doesn't exist.

Change-Id: Iff653cd5b6a7d98ce7e3ead0877cba7b996e9c13
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/574836
Reviewed-by: Lasse Folger <lassefolger@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

compiler/protogen/protogen.go

@@ -143,7 +143,7 @@ type Options struct {
 	//   opts := &protogen.Options{
 	//     ParamFunc: flags.Set,
 	//   }
-	//   protogen.Run(opts, func(p *protogen.Plugin) error {
+	//   opts.Run(func(p *protogen.Plugin) error {
 	//     if *value { ... }
 	//   })
 	ParamFunc func(name, value string) error