ACK_FREQUENCY frame not this

fixes #241

draft-ietf-quic-ack-frequency.md

@@ -434,7 +434,7 @@ On sending an update to the peer's `max_ack_delay`, an endpoint can use this new
 value in later computations of its Probe Timeout (PTO) period; see {{Section 5.2.1
 of QUIC-RECOVERY}}.
 
-Until the packet carrying this frame is acknowledged, the endpoint MUST use the
+Until the packet carrying the ACK_FREQUENCY frame is acknowledged, the endpoint MUST use the
 greater of the current `max_ack_delay` and the value that is in flight when
 computing the PTO period. Doing so avoids spurious PTOs that can be caused by an
 update that decreases the peer's `max_ack_delay`.