cmd/geth: update copyright year (#20512)

Update copyright from 2013-2019 to 2013-2020

cmd/geth/main.go

@@ -194,7 +194,7 @@ func init() {
 	// Initialize the CLI app and start Geth
 	app.Action = geth
 	app.HideVersion = true // we have a command to print the version
-	app.Copyright = "Copyright 2013-2019 The go-ethereum Authors"
+	app.Copyright = "Copyright 2013-2020 The go-ethereum Authors"
 	app.Commands = []cli.Command{
 		// See chaincmd.go:
 		initCommand,