Fix the outdated link of google's java style guide

[tianyin]

Hi,

In CONTRIBUTING.md, the link of Google Java style guide is outdated (given a 404 error code).
The current Java style guide is at
https://google.github.io/styleguide/javaguide.html

(used by the google/guava project
https://github.com/google/guava/blob/master/CONTRIBUTING.md)

This PR is a simple update for the link.

[dimo414]

Thanks for the contribution. I notice you submitted the same fix to the Guice repo; just in case you're looking for other instances, it'll be easier for us to fix internally and push out than for you to send separate pull requests to each project you find.

[tianyin]

@dimo414 thanks! i'm just looking at truth, jimfs, and guice, and find they're in common.

feel free to ignore any of these PRs.
(sorry for the duplicates!)

[dimo414]

Perfect, thanks. I'll get them fixed (and credit you).