Fix references to https://sites.google.com/site/skiadocs/.

Point them to their equivalent in https://skia.org.

BUG=None
R=hcm@google.com, jcgregorio@google.com
NOTRY=true

Review URL: https://codereview.chromium.org/856353004
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 900cad5..aaa83a5 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -1 +1 @@
-Please see the guidelines for contributing code at https://sites.google.com/site/skiadocs/developer-documentation/contributing-code
+Please see the guidelines for contributing code at https://skia.org/dev/contrib/