os: fix documentation for Executable on OpenBSD

Executable on OpenBSD now uses Args[0] so procfs is no longer
required.

Change-Id: I4155ac76f8909499783e876e92ee4f13a35b47dd
Reviewed-on: https://go-review.googlesource.com/46211Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/executable.go

@@ -16,8 +16,7 @@ package os
 // The main use case is finding resources located relative to an
 // executable.
 //
-// Executable is not supported on nacl or OpenBSD (unless procfs is
-// mounted.)
+// Executable is not supported on nacl.
 func Executable() (string, error) {
 	return executable()
 }