Correct a misleading in docstring

Testcase in test_extensions gets api-extensions from section of
[network-feature-enabled] instead of [network]. This is correcting
it and making the docstring more explicit.

Change-Id: I1e25169cc7819e1571bb75522ea3ab5035459bb1
diff --git a/tempest/api/network/test_extensions.py b/tempest/api/network/test_extensions.py
index 2c981a1..84150b4 100644
--- a/tempest/api/network/test_extensions.py
+++ b/tempest/api/network/test_extensions.py
@@ -23,9 +23,9 @@
 
         List all available extensions
 
-    v2.0 of the Neutron API is assumed. It is also assumed that the following
-    options are defined in the [network] section of etc/tempest.conf:
-
+    v2.0 of the Neutron API is assumed. It is also assumed that api-extensions
+    option is defined in the [network-feature-enabled] section of
+    etc/tempest.conf.
     """
 
     @test.attr(type='smoke')