genautocomplete: rename to completion with alias to old name

This brings it into line with cobra's naming scheme and stops cobra
writing another "completion" command which doesn't work as well which
confuses users.

See: https://forum.rclone.org/t/rclone-genautocomplete-bash-vs-rclone-completion-bash-neither-works-fully/38431

cmd/genautocomplete/genautocomplete.go

@@ -11,7 +11,7 @@ func init() {
 }
 
 var completionDefinition = &cobra.Command{
-	Use:   "genautocomplete [shell]",
+	Use:   "completion [shell]",
 	Short: `Output completion script for a given shell.`,
 	Long: `
 Generates a shell completion script for rclone.
@@ -20,4 +20,5 @@ Run with ` + "`--help`" + ` to list the supported shells.
 	Annotations: map[string]string{
 		"versionIntroduced": "v1.33",
 	},
+	Aliases: []string{"genautocomplete"},
 }