Fixed invalid doc references.

src/Avalonia.Controls/AppBuilderBase.cs

@@ -55,8 +55,7 @@ namespace Avalonia.Controls
         public Action<TAppBuilder> AfterSetupCallback { get; private set; } = builder => { };
 
         /// <summary>
-        /// Gets or sets a method to call before <see cref="Start{TMainWindow}"/> is called on the
-        /// <see cref="Application"/>.
+        /// Gets or sets a method to call before Startis called on the <see cref="Application"/>.
         /// </summary>
         public Action<TAppBuilder> BeforeStartCallback { get; private set; } = builder => { };
 
@@ -94,8 +93,7 @@ namespace Avalonia.Controls
         protected TAppBuilder Self => (TAppBuilder) this;
 
         /// <summary>
-        /// Registers a callback to call before <see cref="Start{TMainWindow}"/> is called on the
-        /// <see cref="Application"/>.
+        /// Registers a callback to call before Start is called on the <see cref="Application"/>.
         /// </summary>
         /// <param name="callback">The callback.</param>
         /// <returns>An <typeparamref name="TAppBuilder"/> instance.</returns>