C: add Filippo Valsorda's @golang.org email

Change-Id: I3758a4350b600af304b2cff7ad59c7368a01ab5c
Reviewed-on: https://go-review.googlesource.com/100215Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTORS

@@ -523,7 +523,7 @@ Fedor Indutny <fedor@indutny.com>
 Felipe Oliveira <felipeweb.programador@gmail.com>
 Felix Geisendörfer <haimuiba@gmail.com>
 Filip Gruszczyński <gruszczy@gmail.com>
-Filippo Valsorda <filippo@cloudflare.com> <hi@filippo.io>
+Filippo Valsorda <filippo@golang.org> <filippo@cloudflare.com> <hi@filippo.io>
 Firmansyah Adiputra <frm.adiputra@gmail.com>
 Florian Uekermann <florian@uekermann-online.de> <f1@uekermann-online.de>
 Florian Weimer <fw@deneb.enyo.de>