Fix typo in dtrace_tcp(4)

Using args[2]->tcps_state as-documented results in error:
operator -> cannot be applied to pointer to type "void"

This error is accurate as the synopsis for tcp:::state-change is:
tcp:::state-change(void *, csinfo_t *, void *, tcpsinfo_t *, void *,

tcplsinfo_t *);

args[2] refers to the third argument which is always NULL (as-
documented). The to-state for the TCP connection state transition is
actually in the fourth argument, args[3]->tcps_state.