wpf.md Fix MVVM Example (#51)

Author: suhanlim

content/quickstart/wpf.md

@@ -61,7 +61,7 @@ WPF applications may be created using MVVM ([Model–view–viewmodel](https://e
 
 **View:** Add a `ContentControl` instead of `WpfPlot` in the layout
 ```xml
-    <ContentControl Content={Binding PlotControl, Mode=OneTime}/>
+    <ContentControl Content="{Binding PlotControl, Mode=OneTime}"/>
 ```
 
 **View Model:** Add a `PlotControl` property
@@ -87,4 +87,4 @@ void Plot()
 
 * Plot updates would most likely be done as a reaction to an event or modification of an other property. 
 
-* When to update the plot depends on your use-case. Note that rendering large amounts of data frequently may negatively impact performance.
+* When to update the plot depends on your use-case. Note that rendering large amounts of data frequently may negatively impact performance.