Fix link to dirty bit diagram.

Also add a missing arrow from VertexArray to Context.

Bug: angleproject:3002
Change-Id: I2a30df0bb1c9681b4cf5178b75f1264ddf4ca2e2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592065
Reviewed-by: Jamie Madill <jmadill@chromium.org>

doc/DirtyBits.md

@@ -87,7 +87,7 @@ framebuffer's completeness. On subsequent draw calls we skip re-validation at mi
 
 See the below diagram for the dependency relations between Subjects and Observers.
 
-![State Change Notification Flow](https://raw.githubusercontent.com/google/angle/master/doc/img/StateNotificationFlow.svg?sanitize=true)
+![State Change Notification Flow](https://raw.githubusercontent.com/google/angle/master/doc/img/StateChangeNotificationFlow.svg?sanitize=true)
 
 ## Back-end specific Optimizations