fix TabStripItem markup

Author: stevemonaco

docs/reference/controls/tabstrip.md

@@ -9,7 +9,7 @@ import TabStripNavigation from '/img/reference/controls/tabstrip/tabstrip-naviga
 
 Displays a strip of tab headers. You can use this control as a horizontal menu.
 
-The `TabStrip` is composed of `TabStripItem` children. `TabStripItems` are displayed in order of appearance and can be created by either: 1. a collection of `TabStripItem`s or 2. generated by `ItemsSource`.
+The `TabStrip` is composed of `TabStripItem` children. `TabStripItem`s are displayed in order of appearance and can be created by either: 1. a collection of `TabStripItem`s or 2. generated by `ItemsSource`.
 
 Unlike `TabControl`, `TabStrip` is not responsible for presenting the content of the selected tab. Instead, `TabStrip` requires developers to respond to `SelectionChanged` events or `SelectedItem` property changes and then present the desired content with a separate control, such as `ContentControl`. This gives developers full control over the displayed content and enables use cases such as custom view caching.