Update DispatcherPriority.xml (#4476)

Corrected a typo

Author: Patrick8639

xml/System.Windows.Threading/DispatcherPriority.xml

@@ -22,7 +22,7 @@
 ## Remarks  
  There is no specific system state or "idleness" corresponding to `ApplicationIdle` and `ContextIdle` in the current version of WPF. Both are still valid priorities; an operation with a priority of `ContextIdle` has a higher priority than an operation with a priority of `SystemIdle`.  
   
- The <xref:System.Windows.Threading.Dispatcher> does not throttle the execution of an operation if it exceeds a certain percentage of the CPU. One way to throttle a process it to use a timer.  
+ The <xref:System.Windows.Threading.Dispatcher> does not throttle the execution of an operation if it exceeds a certain percentage of the CPU. One way to throttle a process is to use a timer.  
   
  If an operation is posted using <xref:System.Windows.Threading.Dispatcher.Invoke%2A?displayProperty=nameWithType> on its own <xref:System.Windows.Threading.Dispatcher> at a priority of `Send`, the operation bypasses the queue and is immediately executed.