crypto/x509: correct field name in comment
crypto/x509: correct field name in comment

Fixes issue 8936.

LGTM=bradfitz
R=agl, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/152590043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go