correct length -> count (#9510)

docs/api/core/BufferAttribute.html

@@ -34,7 +34,7 @@
 		attribute is storing a 3-component vector (such as a position, normal, or color), then itemSize should be 3.
 		</div>
 
-		<h3>[property:Integer length]</h3>
+		<h3>[property:Integer count]</h3>
 		<div>
 		Gives the total number of elements in the array.
 		</div>