crypto/x509: clarify accepted keys for MarshalPKCS8PrivateKey

Fixes #24413.

Change-Id: I265088c9ddc624cb3b3132087cc3d4baf95d2777
Reviewed-on: https://go-review.googlesource.com/100839Reviewed-by: Filippo Valsorda <filippo@golang.org>

src/crypto/x509/pkcs8.go

@@ -56,7 +56,7 @@ func ParsePKCS8PrivateKey(der []byte) (key interface{}, err error) {
 }
 
 // MarshalPKCS8PrivateKey converts a private key to PKCS#8 encoded form.
-// The following key types are supported: *rsa.PrivateKey, *ecdsa.PublicKey.
+// The following key types are supported: *rsa.PrivateKey, *ecdsa.PrivateKey.
 // Unsupported key types result in an error.
 //
 // See RFC 5208.