修正 body 参数

[big-dream]

按照文档格式，应该是：

<CompleteMultipartUpload>
    <Part>
        <PartNumber>integer</PartNumber>
        <ETag>string</ETag>
    </Part>
    <Part>
        <PartNumber>integer</PartNumber>
        <ETag>string</ETag>
    </Part>
</CompleteMultipartUpload>

目前是：

<xml>
    <item>
        <PartNumber>integer</PartNumber>
        <ETag>string</ETag>
    </item>
    <item>
        <PartNumber>integer</PartNumber>
        <ETag>string</ETag>
    </item>
</xml>

导致调用接口时，报：

<?xml version='1.0' encoding='utf-8' ?>
<Error>
        <Code>MalformedXML</Code>
        <Message>The XML you provided was not well-formed or did not validate against our published schema.</Message>
        <Resource>/files/mariadb/wampserver3_x64_addon_mariadb10.11.5.exe</Resource>
        <RequestId>NjU1YjFiYTVfNjJkMmMwYl8xNjE0X2U2MDM5Y2U=</RequestId>
        <TraceId>OGVmYzZiMmQzYjA2OWNhODk0NTRkMTBiOWVmMDAxODc0OWRkZjk0ZDM1NmI1M2E2MTRlY2MzZDhmNmI5MWI1OTY4OGQ5OWY4YWFhNjAzOTkyNDJhZmQyOTk1YWVmOWFlZjBkMDIxYTM5ZWI1ODVlNWY4ZmE0NTkzOTRjOTc0YTg=</TraceId>
</Error>