docs: TAP_CONNECT extras flag size seems to be 4 bytes, not 8

Been playing with the tap stream commands, as originally spec'ed here...

https://gist.github.com/23623b951fedc9a56d24

Sending a TAP_CONNECT with an 8-byte extras of TAP_FLAG_DUMP doesn't seem to work (where the 8-bytes extras size seems to be what the gist specifies). That is, the memcached/ep-engine server never closes the connection.

However, sending a TAP_CONNECT cmd with a 4-byte extras with TAP_FLAG_DUMP does work. In this case the memcached/ep-engine server does close the connection.

32 bits (instead of 64) should be enough for everyone.