Update css file name in conf.py

ribbon.css is the correct name of the css file.

Issue-ID: CIMAN-376
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
Change-Id: I2bbb525350252c28e8f6cab9848c504d7bbae630
diff --git a/docs/conf.py b/docs/conf.py
index 8f40e8b..5371015 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -12,4 +12,4 @@
 html_last_updated_fmt = '%d-%b-%y %H:%M'
 
 def setup(app):
-    app.add_stylesheet("css/ribbon_onap.css")
+    app.add_stylesheet("css/ribbon.css")