update viewport variable with new underscore format

Change-Id: I68d9e4a57273d86adb9fa2721dc2ce1dbe3ce937
diff --git a/docs/html/design/index.jd b/docs/html/design/index.jd
index c687f33..7c7c5d9 100644
--- a/docs/html/design/index.jd
+++ b/docs/html/design/index.jd
@@ -1,5 +1,5 @@
 page.title=Design
-page.viewport-width=970
+page.viewport_width=970
 header.hide=1
 footer.hide=1
 @jd:body