Reworded sentence about showAsAction in topbar button doc (#5147)

Fixed minor linguistic mistake

docs/docs/topBar-buttons.md

 
 ## Android showAsAction
 
-It is keyword witch configure when and how this item should appear as an action item in the app bar. 
+The keyword `showAsAction` configures when and how an item should appear as an action item in the app bar. 
 A menu item can appear as an action item only when the activity includes an app bar.
 
 * `always`