Corrected capitalization, corrected to American spellings and typos (#30685)

* 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

guide/english/javascript/arrow-functions/index.md

@@ -65,7 +65,7 @@ function Person() {
 }
 ```
 
-An arrow function doesn't define it's own `this` value, it inherits `this` from the enclosing function:
+An arrow function doesn't define its own `this` value, it inherits `this` from the enclosing function:
 
 ```javascript
 // ES6 syntax