housekeeping: Correct ReactiveWindow usage example

[worldbeater]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

This is documentation improvement. Related to #1888

What is the current behavior? (You can also link to an open issue here)

When bootstrapping a new WPF project with ReactiveUI, I copy-pasted XAML markup snippet from API docs page and it didn't compile due to missing xmlns directives and wrong x:Class syntax.

What is the new behavior (if this is a feature change)?

Now, the sample compiles, and uses correct syntax and correct xmlns directives.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #1899 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1899   +/-   ##
======================================
  Coverage    66.4%   66.4%           
======================================
  Files         110     110           
  Lines        4387    4387           
  Branches      579     579           
======================================
  Hits         2913    2913           
  Misses       1305    1305           
  Partials      169     169

Impacted Files	Coverage Δ
src/ReactiveUI.Wpf/ReactiveWindow.cs	0% <ø> (ø)	⬆️

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