Update gridstack-extra.css in README to point to 2.0.0

Author: jtpio

README.md

@@ -193,7 +193,7 @@ GridStack.init( {column: N} );
 
 2) include `gridstack-extra.css` if **N < 12** (else custom CSS - see next). Without these, things will not render/work correctly.
 ```html
-<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/gridstack@1.1.2/dist/gridstack-extra.css"/>
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/gridstack@2.0.0/dist/gridstack-extra.css"/>
 
 <div class="grid-stack grid-stack-N">...</div>
 ```