more docs fixes

Author: Azq2

README.md

@@ -1174,7 +1174,7 @@ print $text_node->html;                # new>plain>text
 
     ```perl
     my $html = $tree->document->html;
-    my $html = $node->document->outerHTML;
+    my $html = $tree->document->outerHTML;
     ```
 
     Example:

lib/HTML5/DOM.pod

@@ -1098,7 +1098,7 @@ Example:
 HTML serialization of entire document
 
   my $html = $tree->document->html;
-  my $html = $node->document->outerHTML;
+  my $html = $tree->document->outerHTML;
 
 Example: