Fix ext alias in revision API test

This test wasn't ever executed because it was using
'external-network' for the external network alias when
the appropirate alias is 'external-net'.

Change-Id: I41b3c282af3f4d64af8404622a88a78f41f71ada
diff --git a/neutron/tests/tempest/api/test_revisions.py b/neutron/tests/tempest/api/test_revisions.py
index cdcb367..e0419d9 100644
--- a/neutron/tests/tempest/api/test_revisions.py
+++ b/neutron/tests/tempest/api/test_revisions.py
@@ -279,7 +279,7 @@
                            router['revision_number'])
 
     @test.idempotent_id('90743b00-b0e2-40e4-9524-1c884fe3ef23')
-    @test.requires_ext(extension="external-network", service="network")
+    @test.requires_ext(extension="external-net", service="network")
     @test.requires_ext(extension="auto-allocated-topology", service="network")
     @test.requires_ext(extension="subnet_allocation", service="network")
     @test.requires_ext(extension="router", service="network")