Fix index.rst in order to fix doc building

* The name of the doc was contributing.rst not contribute.rst which
  leads to failure of doc build process during rpm packaging in RDO.
  It fixes the same.

Change-Id: I2a90bf6946e1e6dfd26f087210bf20d87f30e0b5
diff --git a/doc/source/index.rst b/doc/source/index.rst
index cc2c1aa..657cd89 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -14,7 +14,7 @@
 
    readme
    installation
-   contributor
+   contributing
 
 Indices and tables
 ==================