Modify comments in xxx_name_length_exceeds_256

The testcase name test_create_server_name_length_exceeds_256 and
test_update_server_name_length_exceeds_256 are not correct, because
the max name length is 255, not 256.

Because now we are unable to modify the testnames in defcore because
alias is not supported in older json files,
https://github.com/openstack/defcore/blob/16668dddea39725088040fdef5404a656b24a606/2015.03.json,
so here we just modify the wrong comment to make the code less
confusing.

Change-Id: I67d5227f308e957e7eabd6230a6b9df17540eb5d
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index f66bc72..2b4cee7 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -209,7 +209,7 @@
     @test.attr(type=['negative'])
     @test.idempotent_id('c3e0fb12-07fc-4d76-a22e-37409887afe8')
     def test_create_server_name_length_exceeds_256(self):
-        # Create a server with name length exceeding 256 characters
+        # Create a server with name length exceeding 255 characters
 
         server_name = 'a' * 256
         self.assertRaises(lib_exc.BadRequest,