Tutorial: rename "Data" to "Corpus Data" in navigation bar and headline

Change-Id: I98a7ad482008972f73180529872dfaf656d33eae
diff --git a/templates/doc/navigation.json b/templates/doc/navigation.json
index 3719190..a904c16 100644
--- a/templates/doc/navigation.json
+++ b/templates/doc/navigation.json
@@ -84,7 +84,7 @@
     ]
   },
   {
-    "title": "Data",
+    "title": "Corpus Data",
     "id": "data",
     "class": "folded",
     "items": [