Revert back to using [heat_plugin] section in tempest.conf

As we're now installing the plugin and running the tests in
a venv at the gate, there is no conflict.

Change-Id: I63dccc94d6954872257062b12114fab07a4e0873
diff --git a/README.rst b/README.rst
index bf9bba5..7206991 100644
--- a/README.rst
+++ b/README.rst
@@ -23,6 +23,6 @@
 
     tempest run --regex heat_tempest_plugin.tests
 
-If custom configuration is required, edit the [orchestration_plugin] section of
+If custom configuration is required, edit the [heat_plugin] section of
 
     $DEST/tempest/etc/tempest.conf