tcp: spelling s/plugable/pluggable

Signed-off-by: NFabian Frederick <fabf@skynet.be>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_cong.c

 /*
- * Plugable TCP congestion control support and newReno
+ * Pluggable TCP congestion control support and newReno
  * congestion control.
  * Based on ideas from I/O scheduler support and Web100.
  *