ipv4: fix typo

Change-Id: Iaec7505bea3ea29cd58147b6057f03a4c7f0861c
Reviewed-on: https://go-review.googlesource.com/29694
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/doc.go

@@ -97,7 +97,7 @@
 // between the protocol stack within the kernel.  When the application
 // needs a destination address on an incoming packet,
 // SetControlMessage of ipv4.PacketConn is used to enable control
-// message transmissons.
+// message transmissions.
 //
 //	if err := p.SetControlMessage(ipv4.FlagDst, true); err != nil {
 //		// error handling