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 the "Requiring more files" section, the class Hola `puts translator.hi` and then returns nil.  Later, in the same section, the same class definition 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
  * in the "Requiring more files" section, the Hola class has a default `language` argument set to the string `"english"`.  Later in this section, the same class definition has this argument 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.