docs: Typo fix ("performed an down" -> "performed a down")

Needed to make a docs fix to verify the (updated) "how to fix a reference
doc bug" instructions (go/fix-refdocs), so I picked this one. Very low
priority, so if merging would be a hassle we don't need to do it.

Bug: 324457037
Change-Id: I216238d8e0da9fafb1fa5a7531f839b6360b5081
Test: ABTD docs build go/abtd/run/L98000030001930492
diff --git a/core/java/android/accessibilityservice/AccessibilityService.java b/core/java/android/accessibilityservice/AccessibilityService.java
index 2a7dbab..e7e3a85 100644
--- a/core/java/android/accessibilityservice/AccessibilityService.java
+++ b/core/java/android/accessibilityservice/AccessibilityService.java
@@ -364,12 +364,12 @@
     public static final int GESTURE_SWIPE_UP_AND_RIGHT = 14;
 
     /**
-     * The user has performed an down and left gesture on the touch screen.
+     * The user has performed a down and left gesture on the touch screen.
      */
     public static final int GESTURE_SWIPE_DOWN_AND_LEFT = 15;
 
     /**
-     * The user has performed an down and right gesture on the touch screen.
+     * The user has performed a down and right gesture on the touch screen.
      */
     public static final int GESTURE_SWIPE_DOWN_AND_RIGHT = 16;