windows: follow convention for generated code comment

Regenerated with go tip to get the generated code comment following the
convention (https://golang.org/s/generatedcode).

Change-Id: Ie1365ecfa532b940bf7cfc8579c4cbda71cf7f76
Reviewed-on: https://go-review.googlesource.com/118817
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/registry/zsyscall_windows.go

-// MACHINE GENERATED BY 'go generate' COMMAND; DO NOT EDIT
+// Code generated by 'go generate'; DO NOT EDIT.
 
 package registry
 

windows/zsyscall_windows.go

-// MACHINE GENERATED BY 'go generate' COMMAND; DO NOT EDIT
+// Code generated by 'go generate'; DO NOT EDIT.
 
 package windows