docs: Update codes in docs to use require('electron')

docs/api/clipboard.md

@@ -4,7 +4,7 @@ The `clipboard` module provides methods to perform copy and paste operations.
 The following example shows how to write a string to the clipboard:
 
 ```javascript
-var clipboard = require('clipboard');
+const clipboard = require('electron').clipboard;
 clipboard.writeText('Example String');
 ```
 
@@ -12,7 +12,6 @@ On X Window systems, there is also a selection clipboard. To manipulate it
 you need to pass `selection` to each method:
 
 ```javascript
-var clipboard = require('clipboard');
 clipboard.writeText('Example String', 'selection');
 console.log(clipboard.readText('selection'));
 ```
@@ -82,7 +81,6 @@ Returns an array of supported formats for the clipboard `type`.
 Returns whether the clipboard supports the format of specified `data`.
 
 ```javascript
-var clipboard = require('clipboard');
 console.log(clipboard.has('<p>selection</p>'));
 ```
 
@@ -102,7 +100,6 @@ Reads `data` from the clipboard.
 * `type` String (optional)
 
 ```javascript
-var clipboard = require('clipboard');
 clipboard.write({text: 'test', html: "<b>test</b>"});
 ```
 Writes `data` to the clipboard.