Fix some minor typos related to the word "accessibility".

ash/system/tray_accessibility.h

Index: ash/system/tray_accessibility.h
diff --git a/ash/system/tray_accessibility.h b/ash/system/tray_accessibility.h
index dfbc22975024a612d49cac877b6a13c7c2c691fd..2120b1de97fd8aca304321800ac166587d0b8fe4 100644
--- a/ash/system/tray_accessibility.h
+++ b/ash/system/tray_accessibility.h
@@ -34,7 +34,7 @@ class ASH_EXPORT AccessibilityObserver {
  public:
   virtual ~AccessibilityObserver() {}
 
-  // Notifies when accessibilty mode changes.
+  // Notifies when accessibility mode changes.
   virtual void OnAccessibilityModeChanged(
       ui::AccessibilityNotificationVisibility notify) = 0;
 };