ipv6: fix typo in package documentation
ipv6: fix typo in package documentation

Change-Id: I94a8b7e876a8cbe4d565bab021f16c1c91f5aa10
Reviewed-on: https://go-review.googlesource.com/12651
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

albertjin/golang-net,dafyddcrosby/net,cmars/tools,w4ngyi/net,muzining/net,cmars/tools,kiip/net,DramaFever/golang.org-x-net,zabawaba99/net,cesanta/goxnet,muzining/net,golang/net,gomini/net,veliamsli/net,raboof/net,cmars/tools,DramaFever/golang.org-x-net,cesanta/goxnet,flowlo/go-net,dafyddcrosby/net,flowlo/go-net,mckinney-subgraph/net,mckinney-subgraph/net,kiip/net,zabawaba99/net,w4ngyi/net,albertjin/golang-net,gomini/net,veliamsli/net,cmars/tools,raboof/net