[DOC] Fix docs build

Change-Id: I7b79921e5e60de4195b30584063022c252ec3378
Issue-ID: OOM-2841
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>
diff --git a/docs/conf.py b/docs/conf.py
index 2b60e51..1126880 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -14,4 +14,4 @@
 exclude_patterns = ['.tox/**']
 
 def setup(app):
-    app.add_stylesheet("css/ribbon_onap.css")
+    app.add_css_file("css/ribbon_onap.css")