Fix typo.

Perhaps there was an if_txrxspeeds option at one point, or a proposal
for that, with that field containing both transmit and receive speeds,
but now we have separate if_txspeed and if_rxspeed options.  Fix a
mention of if_txrxspeeds that may have been left over from that.

draft-ietf-opsawg-pcapng.md

@@ -1171,7 +1171,7 @@ Micro Adapter".
 {: indent='8'}
 if_txspeed:
 : The
-  if_txrxspeeds option is a 64-bit unsigned value
+  if_txspeed option is a 64-bit unsigned value
   indicating the interface transmit speed in bits per
   second.
 {: vspace='0'}