Fix /bin/env vs /usr/bin/env vs nothing

There's no such thing as /bin/env, though I let that slip by because my
distro redirects /bin to /usr/bin.

I believe "#! /bin/sh" is even more standard that "#! /usr/bin/env sh",
and that's what I used everywhere anyway.

change-prefix.sh

@@ -1,4 +1,4 @@
-#! /usr/bin/env sh
+#! /bin/sh
 
 # This file is dual-licensed.  Choose whichever licence you want from
 # the two licences listed below.

doc/doc_check.py

@@ -1,4 +1,4 @@
-#! /bin/env python3
+#! /usr/bin/env python3
 
 # This file is dual-licensed.  Choose whichever licence you want from
 # the two licences listed below.