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/README b/README
index a60bca6..b28fb27 100644
--- a/README
+++ b/README
@@ -1,3 +1,3 @@
 Skia is a complete 2D graphic library for drawing Text, Geometries, and Images.
 
-See full details, and build instructions, at https://sites.google.com/site/skiadocs/home
+See full details, and build instructions, at https://skia.org.