Update Sphere.js

Fix type in `fromJSON()`.

src/math/Sphere.js

@@ -405,7 +405,7 @@ class Sphere {
 	 * Returns a serialized structure of the bounding sphere.
 	 *
 	 * @param {Object} json - The serialized json to set the sphere from.
-	 * @return {Box3} A reference to this bounding sphere.
+	 * @return {Sphere} A reference to this bounding sphere.
 	 */
 	fromJSON( json ) {