internal/wire: use full go run command in go:generate comment (#268)

Change the go:generate command to use the full go run ... command
to ensure the version specified in the current go module is used
instead of the global binary.

Fixes #160

_tutorial/wire_gen.go

@@ -1,6 +1,6 @@
 // Code generated by Wire. DO NOT EDIT.
 
-//go:generate wire
+//go:generate go run github.com/google/wire/cmd/wire
 //+build !wireinject
 
 package main