Below is a rendering of the page up to the first error. (Magento 2)

Question

i am new in magento2 and i am facing this error in api implementation

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This page contains the following errors:
error on line 2 at column 1: Extra content at the end of the document
Below is a rendering of the page up to the first error.
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This is my url : http://127.0.0.1/magento/rest/V1/mastering and this is my webapi.xml file under etc folder:

<?xml version="1.0" ?>
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Webapi:etc/webapi.xsd">
    <route method="GET" url="/V1/mastering">
        <service class="Mastering\SampleModule\Api\ItemRepositoryInterface" method="getList"/>
        <resources>
            <resource ref="anonymous"/>
        </resources>
    </route>
</routes>

if i remove following line

<resource ref="anonymous"/>

error doesn't show neither xml data , can anyone guide me why ?