docs: Fix description of the leaky bucket algorithm in throttle.txt

Commit 0bab0ebb17759c926bd48fd396bd8cbb2c8e4a3e was supposed to fix
a mistake in the description of the leaky bucket algorithm, but the
version that finally landed after the review process was incorrect.

This patch solves that problem and hopefully clarifies the description
a bit better.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
