fix viewport width variable by using underscore instead of hyphen

Change-Id: I866fa7016453b9e242df5e68f26bf94eac672feb
diff --git a/tools/droiddoc/templates-sdk/head_tag.cs b/tools/droiddoc/templates-sdk/head_tag.cs
index 6c804ad..0916b00 100644
--- a/tools/droiddoc/templates-sdk/head_tag.cs
+++ b/tools/droiddoc/templates-sdk/head_tag.cs
@@ -16,8 +16,8 @@
   # END if/else devsite ?>
 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
 <meta name="viewport" content="width=<?cs
-  if:page.viewport-width ?><?cs
-    var:page.viewport-width ?><?cs
+  if:page.viewport_width ?><?cs
+    var:page.viewport_width ?><?cs
   else ?>device-width<?cs /if ?>" />
 <?cs
   if:page.metaDescription ?>