Source code comment cleanup

Simple change making the article agree with its noun.  Twice.

TBR=yigu@chromium.org

Change-Id: I113e3ac0f7700e57c9df3b95a2d7ab155ed5542f
Reviewed-on: https://chromium-review.googlesource.com/c/1354228Reviewed-by: Peter Mayo <petermayo@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612189}

cc/animation/keyframe_effect.h

@@ -27,8 +27,8 @@ struct PropertyAnimationState;
 
 typedef size_t KeyframeEffectId;
 
-// An KeyframeEffect owns a group of KeyframeModels for a single target
-// (identified by a ElementId). It is responsible for managing the
+// A KeyframeEffect owns a group of KeyframeModels for a single target
+// (identified by an ElementId). It is responsible for managing the
 // KeyframeModels' running states (starting, running, paused, etc), as well as
 // ticking the KeyframeModels when it is requested to produce new outputs for a
 // given time.