[docs] Prune import that is never used (#4149)

* [docs] Prune import that is never used

In the Serializing docs, for the `serialize` function for HTML example,
we import `Node` but never use it in the scope of the example. Remove it
to eliminate the possibility for confusion.

* Remove duplicated white line from TypeScript docs

docs/concepts/09-serializing.md

@@ -53,7 +53,7 @@ For example, here's a similar `serialize` function for HTML:
 
 ```js
 import escapeHtml from 'escape-html'
-import { Node, Text } from 'slate'
+import { Text } from 'slate'
 
 const serialize = node => {
   if (Text.isText(node)) {