Fix error in documentation

README.md

@@ -42,7 +42,7 @@ partial class Document extends Node implements \ArrayAccess {
     use DocumentOrElement, NonElementParentNode, ParentNode, XPathEvaluatorBase;
 
     public function __construct(
-        string $source = null,
+        ?string $source = null,
         ?string $charset = null
     );
 

lib/Document.php

@@ -353,7 +353,7 @@ class Document extends Node implements \ArrayAccess {
     }
 
 
-    public function __construct(string $source = null, ?string $charset = null) {
+    public function __construct(?string $source = null, ?string $charset = null) {
         parent::__construct(new InnerDocument($this));
         $this->_implementation = new DOMImplementation($this);