set cord-6.0 to be the branch variable in book.json

Change-Id: I4cecf09d89f3d4d924a774a9004faab74fe3e5ff
diff --git a/book.json b/book.json
index 1317bf7..4421194 100644
--- a/book.json
+++ b/book.json
@@ -5,7 +5,7 @@
     "summary": "SUMMARY.md"
   },
   "variables": {
-    "branch": "master"
+    "branch": "cord-6.0"
   },
   "plugins": [
     "anchorjs",