Fix constant of frame_type IMMEDIATE_ACK

Both the draft and the IANA registry say the current value for
frame_type IMMEDIATE_ACK is 0x1F.

https://www.iana.org/assignments/quic/quic.xhtml
https://datatracker.ietf.org/doc/html/draft-ietf-quic-ack-frequency#section-10.2

Co-Authored-By: Tamas Levai <levait@tmit.bme.hu>

quinn-proto/src/frame.rs

@@ -131,7 +131,7 @@ frame_types! {
     HANDSHAKE_DONE = 0x1e,
     // ACK Frequency
     ACK_FREQUENCY = 0xaf,
-    IMMEDIATE_ACK = 0xac,
+    IMMEDIATE_ACK = 0x1f,
     // DATAGRAM
 }