* Updated index.md to change the grammatical mistakes
In the line no. 6, added an "an" before the word "element". Similar corrections in line no. 8, 9, and 10. In line no. 8, "then" is added in the line to make a proper meaning in completing the sentence. The word "to the" is added before the words such as "top", "left" and "bottom" in line nos. 9 and 10 to make a proper sentence.
* Update index.md
* Added a , before the then statement
* 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
