minor #10831 [Serializer] XmlEncoder example fix (vertisan)

This PR was merged into the 4.2 branch.

Discussion
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[Serializer] XmlEncoder example fix

Currently in example:
```
$encoder = new XmlEncoder();
$encoder->encode(array('foo' => array('@bar' => 'value'), 'qux' => array('#comment' => 'A comment));
```
we'll get `Parse error: syntax error, unexpected '', $serialized); ' (T_CONSTANT_ENCAPSED_STRING), expecting ')'`.
After close `A comment` string and add missing `$format`, it works correctly.
```
$encoder->encode(array('foo' => array('@bar' => 'value'), 'qux' => array('#comment' => 'A comment')), 'xml');
```

Commits
-------

9df531a XmlEncoder example fix

Author: javiereguiluz

components/serializer.rst

@@ -804,7 +804,7 @@ Be aware that this encoder will consider keys beginning with ``@`` as attributes
 the key  ``#comment`` for encoding XML comments::
 
     $encoder = new XmlEncoder();
-    $encoder->encode(array('foo' => array('@bar' => 'value'), 'qux' => array('#comment' => 'A comment));
+    $encoder->encode(array('foo' => array('@bar' => 'value'), 'qux' => array('#comment' => 'A comment')), 'xml');
     // will return:
     // <?xml version="1.0"?>
     // <response>