crypto/x509: correct field name in comment

Fixes golang#8936.

LGTM=bradfitz
R=agl, bradfitz
CC=golang-codereviews
https://golang.org/cl/152590043

src/crypto/x509/x509.go

@@ -1670,7 +1670,7 @@ var oidExtensionRequest = asn1.ObjectIdentifier{1, 2, 840, 113549, 1, 9, 14}
 
 // CreateCertificateRequest creates a new certificate based on a template. The
 // following members of template are used: Subject, Attributes,
-// SignatureAlgorithm, Extension, DNSNames, EmailAddresses, and IPAddresses.
+// SignatureAlgorithm, Extensions, DNSNames, EmailAddresses, and IPAddresses.
 // The private key is the private key of the signer.
 //
 // The returned slice is the certificate request in DER encoding.