docs: make src/docs/copy.sh shebang more portable

`/bin/bash` is not guaranteed to exist on all systems. Replacing the
shebang with `/usr/bin/env bash` looks up the location of bash on such
systems.

src/doc/copy.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 html_path=../../_doc/_html
 # get name of internal library
 lib_name=$(ls $html_path | grep kind2dev)