derp: update DERP acronym expansion
Makes the package description consistent with other documentation.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
-// Package derp implements DERP, the Detour Encrypted Routing Protocol.
+// Package derp implements the Designated Encrypted Relay for Packets (DERP)
+// protocol.
//
// DERP routes packets to clients using curve25519 keys as addresses.
//
