Change the TYPE_PRNG_SEED length to 24.

This matches what the code actually sends.  It's shorter than the
ScrambleSuit PRNG seed, but that's because the SipHash-2-4 based
Hash_DRBG has 24 bytes of internal state (key + initial output).

doc/obfs4-spec.txt

@@ -104,7 +104,7 @@
 
        Length of the ntor AUTH tag (A HMAC-SHA256 digest).
 
-     InlineSeedFrameLength = 53
+     InlineSeedFrameLength = 45
 
        Length of a unpadded TYPE_PRNG_SEED frame.
 
@@ -144,7 +144,7 @@
 
    The amount of padding is chosen such that the smallest possible request and
    response (requests and responses with the minimum amount of padding) are
-   equal in size.  For details on the InlineSeedFrameLength, see section 7.
+   equal in size.  For details on the InlineSeedFrameLength, see section 6.
 
    The client handshake process is as follows.
 
@@ -282,7 +282,7 @@
      TYPE_PRNG_SEED (0x01):
 
          The entire payload is to be treated as seeding material for the
-         protocol polymorphism PRNG.  The format is 32 bytes of seeding
+         protocol polymorphism PRNG.  The format is 24 bytes of seeding
          material.
 
    Implementations SHOULD ignore unknown packet types for the purposes of