docs: Update the link for Google's Style Guide

[RohanTalip]

... from the out-of-date http://checkstyle.sourceforge.net/reports/google-java-style.html (last updated 2014-03-21)
... to https://google.github.io/styleguide/javaguide.html (last updated 2018-05-22)

See also https://github.com/google/styleguide/blob/gh-pages/javaguide.html

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the development branch.
  • Not applicable; there is no development branch yet.
• I have added tests that prove my fix is effective or that my feature works
  • Not applicable.
• I have added necessary documentation about the functionality in the appropriate .md file
  • Not applicable.
• I have added in line documentation to the code I modified
  • Not applicable.

[pavan-kalyan]

Looks good to me.

[thinkingserious]

Hello @RohanTalip,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX