fixed broken video links
fixed broken video links

Change-Id: I32935cb638dffeaf6eee941c7725e93f956e267c

GoogleCloudPlatform/tensorflow-without-a-phd,GoogleCloudPlatform/tensorflow-without-a-phd,GoogleCloudPlatform/tensorflow-without-a-phd,GoogleCloudPlatform/tensorflow-without-a-phd