ipv6: fix typo

Change-Id: I2e0a8f0e344a2fc31b3d40e0b9f3812dbbd75569
Reviewed-on: https://go-review.googlesource.com/29695
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv6/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 ipv6.PacketConn is used to enable control
-// message transmissons.
+// message transmissions.
 //
 //	if err := p.SetControlMessage(ipv6.FlagDst, true); err != nil {
 //		// error handling