Increment theorem counter after Lemma 6.5.4.

There used to be a numbered equation (6.5.5) in the proof of 6.5.4,
but it was removed by commit 8f54d16.  Incrementing the counter anyway
ensures that if we ever add new theorems or equations at the end of
section 6.5, they will start with 6.5.6, preventing any confusion.