Fix balancedParens example for REPLICATE's new automatic rewrite

All l/case short simp tacs use LET_ss + ARITH_ss

This means that fs and rfs pick up LET_ss and ARITH_ss, making them
indistinguishable from the 'l' versions.

balanced parentheses example

Inspired by an extra exercise at the Isabelle tutorial (Nanjing, 2015).
My version in Isabelle is about 40 lines longer. (This version includes
input from Michael Norrish to make it easier to write and more concise.)