doc: don't imply that the new HTTP status 451 is from RFC 6585
doc: don't imply that the new HTTP status 451 is from RFC 6585

From twitter bug report: https://twitter.com/ox/status/692737249411207168

Change-Id: Ic5f4eeb00d705217542db558edc25e206f6b640d
Reviewed-on: https://go-review.googlesource.com/19050
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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