ipv6: fix a typo

Change-Id: I70c1953d811234a78c88d073ff2a0001fbc9b6bf
Reviewed-on: https://go-review.googlesource.com/126640
Reviewed-by: Matt Layher <mdlayher@gmail.com>

ipv6/doc.go

@@ -56,7 +56,7 @@
 // Multicasting
 //
 // The options for multicasting are available for net.UDPConn and
-// net.IPconn which are created as network connections that use the
+// net.IPConn which are created as network connections that use the
 // IPv6 transport. A few network facilities must be prepared before
 // you begin multicasting, at a minimum joining network interfaces and
 // multicast groups.