x/crypto/pkcs12: fix typo in struct tag

No test, though, because I don't know this code. Sad that it passes
with the typo, though.

Fixes golang/go#13855

Change-Id: I1170baa0f94ae4b671f5ca654cb7dcc393d1cc4d
Reviewed-on: https://go-review.googlesource.com/18363
Reviewed-by: Adam Langley <agl@golang.org>

pkcs12/pkcs12.go

@@ -65,7 +65,7 @@ type safeBag struct {
 
 type pkcs12Attribute struct {
 	Id    asn1.ObjectIdentifier
-	Value asn1.RawValue `ans1:"set"`
+	Value asn1.RawValue `asn1:"set"`
 }
 
 type encryptedPrivateKeyInfo struct {