go-tour: use tabs for indentation

LGTM=adg
R=adg, kevlar
CC=golang-codereviews
https://golang.org/cl/145160043
diff --git a/static/js/values.js b/static/js/values.js
index a354083..8ec323c 100644
--- a/static/js/values.js
+++ b/static/js/values.js
@@ -56,6 +56,8 @@
         lineNumbers: true,
         autofocus: true,
         indentWithTabs: true,
+        indentUnit: 4,
+        tabSize: 4,
         lineWrapping: true,
         extraKeys: {
             'Shift-Enter': function() {