unix: drop reference to mkunix.pl from comment

mkunix.pl doesn't exist in x/sys/unix, so drop the reference to it from
the _zero godoc comment.

Change-Id: I7a638a5248b72c4dffc83da1514e5d861c532127
Reviewed-on: https://go-review.googlesource.com/c/sys/+/174858
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall.go

@@ -50,5 +50,4 @@ func BytePtrFromString(s string) (*byte, error) {
 }
 
 // Single-word zero for use when we need a valid pointer to 0 bytes.
-// See mkunix.pl.
 var _zero uintptr