ipv4: fix typo in comment
ipv4: fix typo in comment

Change-Id: I60db66b5bbe25b434d8ec1fbc031438a16720dfa
GitHub-Last-Rev: 152f0ce49bf929a7859818f19c8f76da9c844e27
GitHub-Pull-Request: golang/net#30
Reviewed-on: https://go-review.googlesource.com/c/163865
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net