Minor Python3 script fixups.

Make some scripts executable and use the shebang line recommended at
https://docs.python.org/3/using/unix.html#miscellaneous

doc/extcap_example.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # Copyright 2014 Roland Knall <rknall [AT] gmail.com>
 #

epan/wslua/make-reg.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # make-reg.py
 # Registration Macros Generator

tools/generate_authors.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 #
 # Generate the AUTHORS file combining existing AUTHORS file with