fix shellcheck errors in 2018/burton2/mkkeywords.sh

2018/burton2/mkkeywords.sh

@@ -1,3 +1,3 @@
-#!/bin/sh
+#!/usr/bin/env bash
 awk '/^[^$]/{l=length;m=m>l?m:l;a[l]=a[l]$1}
-     END{printf"\"";for(i=1;i<=m;++i)printf" "a[i];print"\""}' $*
+     END{printf"\"";for(i=1;i<=m;++i)printf" "a[i];print"\""}' "$@"