XML comment fixes.

src/Avalonia.Controls/MenuItem.cs

@@ -421,7 +421,7 @@ namespace Avalonia.Controls
         }
 
         /// <summary>
-        /// Called when the <see cref="Header"/> property changes.
+        /// Called when the <see cref="HeaderedSelectingItemsControl.Header"/> property changes.
         /// </summary>
         /// <param name="e">The property change event.</param>
         private void HeaderChanged(AvaloniaPropertyChangedEventArgs e)

src/Avalonia.Controls/PixelPointEventArgs.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Initializes a new instance of the <see cref="PixelPointEventArgs"/> class.
         /// </summary>
-        /// <param name="point">The <see cref=PixelPoint"/> data.</param>
+        /// <param name="point">The <see cref="PixelPoint"/> data.</param>
         public PixelPointEventArgs(PixelPoint point)
         {
             Point = point;