[FIX] Fix typo in file picker sample source (#1497)

There was a mismatch in the name for TextBlock control in the first
FilePicker sample. Changed name from `OutputTextBlock` to
`PickAFileOutputTextBlock` in the sample source text file.

Author: zateutsch

WinUIGallery/ControlPagesSampleCode/System/FilePickerSample1_cs.txt

@@ -1,7 +1,7 @@
 private async void PickAFileButton_Click(object sender, RoutedEventArgs e)
 {
     // Clear previous returned file name, if it exists, between iterations of this scenario
-    OutputTextBlock.Text = "";
+    PickAFileOutputTextBlock.Text = "";
 
     // Create a file picker
     var openPicker = new Windows.Storage.Pickers.FileOpenPicker();
@@ -29,4 +29,4 @@
     {
         PickAFileOutputTextBlock.Text = "Operation cancelled.";
     }
-}
+}