I made little improvements to the spanish translation. For instance, in "el campo de <code>input<code>" doesn't require the word "de".
Rearranged some words positioning so that the text reads more naturally.
* Translate challenge subtitles and example challenge text to Spanish
* Corrected errors in syntax and punctuation
* Multiple corrections of it/s to its plus other grammar corrections
* Correction and added paragraph to CSS Flex article
* Corrected my own typo
* Corrected capitalization, American spellings and typos