tarantoolctl: fix command in help messages

Currently `tarantoolctl rocks --help` generate such help message:

NAME
	/usr/bin/tarantoolctl - LuaRocks main command-line interface

SYNOPSIS
	/usr/bin/tarantoolctl [<flags...>] [VAR=VALUE]...

This is wrong.

This patch makes the output look like this:

NAME
	/usr/bin/tarantoolctl rocks - LuaRocks main command-line interface

SYNOPSIS
	/usr/bin/tarantoolctl rocks [<flags...>] [VAR=VALUE]...

NO_DOC=bugfix