Fix override-defaults in plugin docs

Docs specify that this file should be override_defaults, when really
devstack looks for override-defaults.

Change-Id: I3900ec4d16ffb48c6969dac5081ea2817536c246
diff --git a/doc/source/plugins.rst b/doc/source/plugins.rst
index b8da7e1..83e5609 100644
--- a/doc/source/plugins.rst
+++ b/doc/source/plugins.rst
@@ -21,12 +21,12 @@
 An external git repository that includes a ``devstack/`` top level
 directory. Inside this directory there can be 3 files.
 
-- ``override_defaults`` - a file containing global variables that
+- ``override-defaults`` - a file containing global variables that
   will be sourced before the lib/* files. This allows the plugin
   to override the defaults that are otherwise set in the lib/*
   files.
 
-  For example, override_defaults may export CINDER_ENABLED_BACKENDS
+  For example, override-defaults may export CINDER_ENABLED_BACKENDS
   to include the plugin-specific storage backend and thus be able
   to override the default lvm only storage backend for Cinder.