Fix shebang in top level Makefile

It had:

    #build authors.html if out of date with author JSON files'!/usr/bin/env make

so it was changed to:

    !/usr/bin/env make

It seems like this was somehow yanked pasted from make help.

Makefile

@@ -1,4 +1,4 @@
-#build authors.html if out of date with author JSON files'!/usr/bin/env make
+#!/usr/bin/env make
 #
 # IOCCC Makefile