Consistency for Hola & Hola::Translator classes

* Two proposed small changes for the make-your-own-gem.md file
  * removing unneeded `puts` in Hola.hi method definition.
    * in "Requiring more files" section, the class Hola `puts translator.hi` and then returns nil.  Later, in the same section, it returns translator.hi directly.  I'm suggesting removing the `puts` in the first example, as it breaks tests later in the guide.
  * using string arguments consistently
    * also in the "Requiring more files" section, the Hola class has a default `language` argument set to "english," a string.  Later, this argument is changed to :english, a symbol. The Hola::Translator class assumes a string `language`, and so both the example of using `Hola.hi` as well as the tests break.