fix modules/zend.i18n.translating.rst

I fixed some grammar issues. Also, using the method addTranslationFile(), it isn't apparent what parameter value is expected for use with PHP arrays. It took me a look into the Loader directory of the Zend\I18n\Translator component to figure it out. I propose you make this one more clear.

Also - as a reminder - this document states that this component supports tmx & xliff, but the file loaders aren't there yet.

This is my first patch ever with github, and I beg for pardon if I made any mistakes. Cheers.