Fixed Stream sample in Additional files documentation

[svick]

The original code had two issues:

• The StreamWriter closes the underlying MemoryStream, so XDocument can't read from it.
• After writing, the MemoryStream stays at its end, so XDocument wouldn't read anything.

This change fixes both issues.

I have also mentioned converting to string using ToString in the description before the example. Though IMHO, in this specific case, calling ToString() and then using XDocument.Parse() instead would be preferable.

[svick]

CC: @tmeschter

[Pilchie]

Tag @dotnet/roslyn-analysis Can you take a look?

[tmeschter]

LGTM.

[mavasani]

👍 Thanks @svick