Update link to Google style guidelines

Google Code is no more. Update the link to a copy of the style guide else
where.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

content/guidelines/_index.md

@@ -80,15 +80,10 @@ resources, which can be quite comprehensive and are worth reading.
 
 ## External links
 
-  - [CERT secure C coding
-    guidelines](https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard)
-  - [MISRA C coding guidelines
-    overview](http://en.wikipedia.org/wiki/MISRA_C) (guidelines
-    themselves are non-free)
-  - [GNOME Programming
-    Guidelines](https://developer.gnome.org/programming-guidelines/unstable/)
-  - [Google C++
-    guidelines](https://google-styleguide.googlecode.com/svn/trunk/cppguide.html#General_Naming_Rules)
+  - [CERT secure C coding guidelines](https://www.securecoding.cert.org/confluence/display/seccode/CERT+C+Coding+Standard)
+  - [MISRA C coding guidelines overview](http://en.wikipedia.org/wiki/MISRA_C) (guidelines themselves are non-free)
+  - [GNOME Programming Guidelines](https://developer.gnome.org/programming-guidelines/unstable/)
+  - [Google C++ guidelines](https://google.github.io/styleguide/cppguide.html#General_Naming_Rules)
     — useful general advice, but often not specific to GLib-style C, and
     sometimes contradictory to the GNOME Programming Guidelines, which
     are preferred