acme: generate correct ES512 signatures

The correct curve name for ES512 signatures is P-521, not P-512.
Author: Matthew Endsley <mendsley@gmail.com>.

Fixes golang/go#18357.

Change-Id: I8d9ba046780921a9e1a5eaa947b9bcbd12e60e34
Reviewed-on: https://go-review.googlesource.com/34555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/acme/jws.go b/acme/jws.go
index 49ba313..6cbca25 100644
--- a/acme/jws.go
+++ b/acme/jws.go
@@ -134,7 +134,7 @@
 			return "ES256", crypto.SHA256
 		case "P-384":
 			return "ES384", crypto.SHA384
-		case "P-512":
+		case "P-521":
 			return "ES512", crypto.SHA512
 		}
 	}