Update free-programming-books-th.md (#7598)

* Update free-programming-books-th.md

additional Java exercise.

* lint: leave 2 blank lines between listings and next heading

Co-authored-by: Biswadeep Purkayastha <98874428+metabiswadeep@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: Biswadeep Purkayastha <98874428+metabiswadeep@users.noreply.github.com>