cmd, geth: CLI help fixes (#22220)

* cmd, geth: Reflect command being optional - closes 22218

* cmd, geth: Set current year to 2021

internal/flags/helpers.go

@@ -51,10 +51,10 @@ OPTIONS:
 	AppHelpTemplate = `NAME:
    {{.App.Name}} - {{.App.Usage}}
 
-   Copyright 2013-2019 The go-ethereum Authors
+   Copyright 2013-2021 The go-ethereum Authors
 
 USAGE:
-   {{.App.HelpName}} [options]{{if .App.Commands}} command [command options]{{end}} {{if .App.ArgsUsage}}{{.App.ArgsUsage}}{{else}}[arguments...]{{end}}
+   {{.App.HelpName}} [options]{{if .App.Commands}} [command] [command options]{{end}} {{if .App.ArgsUsage}}{{.App.ArgsUsage}}{{else}}[arguments...]{{end}}
    {{if .App.Version}}
 VERSION:
    {{.App.Version}}
@@ -77,7 +77,7 @@ COPYRIGHT:
 	ClefAppHelpTemplate = `NAME:
    {{.App.Name}} - {{.App.Usage}}
 
-   Copyright 2013-2019 The go-ethereum Authors
+   Copyright 2013-2021 The go-ethereum Authors
 
 USAGE:
    {{.App.HelpName}} [options]{{if .App.Commands}} command [command options]{{end}} {{if .App.ArgsUsage}}{{.App.ArgsUsage}}{{else}}[arguments...]{{end}}