* Add foreign key definition and example
A very basic foreign key definition and example
* Fix typo as recommended
Line 66: as @nik recommendation
Line67: fix plural nouns
* grammar
* 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