Fix basic grammar mistake

http://data.grammarbook.com/blog/pronouns/1-grammar-error/

docs/api/objects/Group.html

@@ -13,7 +13,7 @@
 		<h1>[name]</h1>
 
 		<div class="desc">
-			This is almost identical to an [page:Object3D Object3D]. It's purpose is to make working
+			This is almost identical to an [page:Object3D Object3D]. Its purpose is to make working
 			with groups of objects syntactically clearer.
 		</div>