📝 Fix code style issue

* Change `var` to `let`.
* Change `function() {}` to `() => {}`.
* Use shorthand function syntax on object notation.
* Remove spaces between object notation brackets.
* Small fixes.

docs/api/web-frame.md

@@ -59,7 +59,7 @@ An example of using [node-spellchecker][spellchecker] as provider:
 
 ```javascript
 webFrame.setSpellCheckProvider('en-US', true, {
-  spellCheck: function(text) {
+  spellCheck(text) {
     return !(require('spellchecker').isMisspelled(text));
   }
 });