Fixed image tag of "Lab 8" by kakakakakku ยท Pull Request #105 ยท openfaas/workshop

Hi, Thank you for awesome workshop ๐Ÿ‘ What I found that image tag of sleep-for is wrong. Because after faas-cli new, latest is default image tag. Fixed a little :) Thanks ๐Ÿณ๐Ÿณ๐Ÿณ