Replace deprecated test.attr with decorators.attr

Tempest has deprecated decorator 'tempest.test.attr' and
moved the same as stable interface to 'tempest.lib.decorators
.attr' in version 16.1.0.

Let's use the stable interface for attr decorator.
This will help Tempest to remove the deprecated decorator.

Change-Id: If72deb25a1438470442b153a41303ca6c7cbd22b
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 60b131b..b5770f2 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -168,7 +168,7 @@
 
            cls.client = cls.os.zones_client
 
-       @test.attr(type='smoke')
+       @decorators.attr(type='smoke')
        @decorators.idempotent_id('fbabd6af-238a-462e-b923-de4d736b90a7')
        def test_create_zone(self):
            LOG.info('Create a zone')
@@ -244,7 +244,7 @@
            pass
 
 
-@test.attr
+@decorators.attr
 ~~~~~~~~~~
 
 The `attr` decorator is used to set test attributes, this is most commonly used
@@ -259,11 +259,11 @@
 .. code-block:: python
 
    class ZonesTest(BaseZonesTest):
-       @test.attr(type='smoke')
+       @decorators.attr(type='smoke')
        def test_create_zone(self):
            pass
 
-       @test.attr(type='slow')
+       @decorators.attr(type='slow')
        def test_something_else(self):
            pass