tutorial: update go_tutorial.txt

should have been included in last CL; no idea why it was not

R=brainman
CC=golang-dev
https://golang.org/cl/4005043

doc/go_tutorial.txt

@@ -482,7 +482,7 @@ something from the directory of installed packages.
 (Also, ''"file.go"'' must be compiled before we can import the
 package.)
 
-Now we can compile and run the program:
+Now we can compile and run the program. On Unix, this would be the result:
 
 	$ 6g file.go                       # compile file package
 	$ 6g helloworld3.go                # compile main package