ed25519: update document to mention RFC 8032

The EdDSA draft has been approved as RFC 8032.

Change-Id: I2c0fba98d63ea51a7bb79acef55b9847cd0fa727
Reviewed-on: https://go-review.googlesource.com/74670
Reviewed-by: Adam Langley <agl@golang.org>

ed25519/ed25519.go

@@ -6,7 +6,7 @@
 // https://ed25519.cr.yp.to/.
 //
 // These functions are also compatible with the “Ed25519” function defined in
-// https://tools.ietf.org/html/draft-irtf-cfrg-eddsa-05.
+// RFC 8032.
 package ed25519
 
 // This code is a port of the public domain, “ref10” implementation of ed25519