crypto/x509: fix typo in comment.
crypto/x509: fix typo in comment.

Fixes issue 6633.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/40720046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang