Fix typo in TranslationRequest#initialize exception

Very quick fix, there was a typo in "No target language provided"

Found it while trying to run this example from the readme (which appears to miss a parameter):
```
# mention that you're submitting HTML (translate calls only)
EasyTranslate.translate("<b>Hello</b>", :html => true) # => "<b>¡Hola</b>"
```

Thanks for your Gem :+1: