Update the `help` text for the `--dump` option

[marco-c]

It currently says Specifies the output file, but actually the --dump option is dumping the AST to stdout, e.g.:

`- {program:121} from (1, 1) to (3, 1) 
   |- {lexical_declaration:134} from (1, 1) to (1, 18) : const x = y => y; 
   |  |- {const:14} from (1, 1) to (1, 6) : const 
   |  |- {variable_declarator:135} from (1, 7) to (1, 17) : x = y => y 
   |  |  |- {identifier:1} from (1, 7) to (1, 8) : x 
   |  |  |- {=:39} from (1, 9) to (1, 10) : = 
   |  |  `- {arrow_function:180} from (1, 11) to (1, 17) : y => y 
   |  |     |- {identifier:1} from (1, 11) to (1, 12) : y 
   |  |     |- {=>:52} from (1, 13) to (1, 15) : => 
   |  |     `- {identifier:1} from (1, 16) to (1, 17) : y 
   |  `- {;:33} from (1, 17) to (1, 18) : ; 
   `- {lexical_declaration:134} from (2, 1) to (2, 20) : const z = (t) => t; 
      |- {const:14} from (2, 1) to (2, 6) : const 
      |- {variable_declarator:135} from (2, 7) to (2, 19) : z = (t) => t 
      |  |- {identifier:1} from (2, 7) to (2, 8) : z 
      |  |- {=:39} from (2, 9) to (2, 10) : = 
      |  `- {arrow_function:180} from (2, 11) to (2, 19) : (t) => t 
      |     |- {formal_parameters:206} from (2, 11) to (2, 14) : (t) 
      |     |  |- {(:19} from (2, 11) to (2, 12) : ( 
      |     |  |- {identifier:1} from (2, 12) to (2, 13) : t 
      |     |  `- {):20} from (2, 13) to (2, 14) : ) 
      |     |- {=>:52} from (2, 15) to (2, 17) : => 
      |     `- {identifier:1} from (2, 18) to (2, 19) : t 
      `- {;:33} from (2, 19) to (2, 20) : ;