Fixed OxyPlot style reference in README.md (#63)

Author: dephonica

README.md

@@ -24,7 +24,7 @@ To use the library, add the following to your `App.xaml`:
         <StyleInclude Source="resm:OxyPlot.Avalonia.Themes.Default.xaml?assembly=OxyPlot.Avalonia"/>
 
         <!-- Add the line below to get OxyPlot UI theme applied in Avalonia 11. -->
-        <StyleInclude Source="avares://AvaloniaEdit/Themes/Simple/AvaloniaEdit.xaml"/>
+        <StyleInclude Source="avares://OxyPlot.Avalonia/Themes/Default.axaml"/>
     </Application.Styles>
 </Application>
 ```