Update link to Google style guide.

Change-Id: I0c9d86f188cd20d256620ccbb46546678714e081
Reviewed-on: https://boringssl-review.googlesource.com/6386
Reviewed-by: Adam Langley <agl@google.com>

STYLE.md

@@ -1,7 +1,7 @@
 # BoringSSL Style Guide
 
 BoringSSL usually follows the
-[Google C++ style guide](https://google-styleguide.googlecode.com/svn/trunk/cppguide.html).
+[Google C++ style guide](https://google.github.io/styleguide/cppguide.html),
 The rest of this document describes differences and clarifications on
 top of the base guide.