Updated the document URL in comments to avoid dead link
Old: http://www.secg.org/download/aid-780/sec1-v2.pdf
New: http://www.secg.org/sec1-v2.pdf

Change-Id: If13d0da4c0e7831b2bd92c45116c2412a2a965f5
Reviewed-on: https://go-review.googlesource.com/11550Reviewed-by: Russ Cox <rsc@golang.org>