Fix typo in WPF setup instructions (#46)

Author: grahas

docs/getting-started/csharp.mdx

@@ -29,7 +29,7 @@ Get started with .NET 5+ (cross-platform) or .NET Framework.
         <TabItem value="wpf" label="WPF">
             Though it is possible to simply put the Velopack bootstrap call within `App.xaml.cs`, we recommend creating a custom `Main` method to avoid any WPF overhead when applying updates.
 
-            First, change the Build Actio of `App.xaml` to `Page` (right-click the file in Solution Explorer, select Properties, and change the Build Action). 
+            First, change the Build Action of `App.xaml` to `Page` (right-click the file in Solution Explorer, select Properties, and change the Build Action). 
             The project file should contain the following lines:
             ```xml
             <ItemGroup>
@@ -139,4 +139,4 @@ Get started with .NET 5+ (cross-platform) or .NET Framework.
 </FancyStep>
 
 
-<Completion />
+<Completion />