Fixed incorrect logic in XmlReader's example (#43799)

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(#43799 is now 3 weeks old without even being triaged, so fixing it myself)

The code sample for XmlReader is incorrect. It relies on the fact that the input XML has some formatting in it. Without formatting the code returns wrong results.

In more details, the issue happens because XElement.ReadFrom() reads the current element and positions the reader at the next element. When the XML has formatting, that next element will be of type Whitespace or smth - and everything works. But without formatting that next element will be the next Child element. Then the reader.Read() happens, which moves the reader to yet another next element. Hence the <Child Key=""02""> tag simply gets skipped.

Here is the code to address the issue.

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docs/standard/linq/stream-xml-fragments-xmlreader.md	How to stream XML fragments from an XmlReader (LINQ to XML)

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