Update README.md for incubator-samza-hello-samza to refer to 0.8

[Roger Hoover]

Hi,

Can someone update the README.md in the hello samza project to link to 0.8
<http://samza.incubator.apache.org/startup/hello-samza/0.8/>?  It's still
pointing to 0.7 docs even though master is not 0.8.

Thanks,

Roger