docs: Update the link for Google's Style Guide (sendgrid#540)

... from the out-of-date http://checkstyle.sourceforge.net/reports/google-java-style.html (last updated 2014-03-21)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

Author: RohanTalip

CONTRIBUTING.md

@@ -155,7 +155,7 @@ Generally, we follow the style guidelines as suggested by the official language.
 
 Please run your code through:
 - [FindBugs](http://findbugs.sourceforge.net/)
-- [CheckStyle](http://checkstyle.sourceforge.net/) with [Google's Java Style Guide](http://checkstyle.sourceforge.net/reports/google-java-style.html).
+- [CheckStyle](http://checkstyle.sourceforge.net/) with [Google's Java Style Guide](https://google.github.io/styleguide/javaguide.html).
 
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