Fix rw explanation in Tactics chapter

[freizl]

For this example

example (a b c : Nat) : a + b + c = a + c + b := by
  rw [Nat.add_assoc, Nat.add_comm b, ← Nat.add_assoc]

This PR fixes a documentation error in the rw tactic explanation in book/TPiL/Tactics.lean.

The text previously said that, without specifying the argument, rewriting
a + (b + c) with Nat.add_comm would produce (b + c) + a.
That is incorrect for this example.

It now correctly states that the rewrite is:
a + (b + c) -> a + (c + b).