DOC : commented out non-existent file

Author: tacaswell

doc/api/index_backend_api.rst

@@ -9,6 +9,6 @@ backends
    backend_qt4agg_api.rst
    backend_wxagg_api.rst
    backend_pdf_api.rst
-   backend_webagg.rst
+..   backend_webagg.rst
    dviread.rst
    type1font.rst