docs: update websockets import var (#165)

Looks like this was missed when updating docs

Author: vasco-santos

README.md

@@ -43,14 +43,14 @@ $ npm i @libp2p/websockets
 ### Constructor properties
 
 ```js
-import WS from '@libp2p/websockets'
+import { WebSockets } from '@libp2p/websockets'
 
 const properties = {
   upgrader,
   filter
 }
 
-const ws = new WS(properties)
+const ws = new WebSockets(properties)
 ```
 
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