anchors: Fix invalid YAML in aliases test cases

Although accepted by PyYAML, `{*x: 4}` is not valid YAML: it should be
noted `{*x : 4}`. The reason is that a colon can be part of an anchor
name. See this comment from Tina Müller for more details:
https://github.com/adrienverge/yamllint/pull/550#discussion_r1155297373

Even if it's not a problem for yamllint, let's fix our tests to include
valid YAML snippets.

tests/rules/test_anchors.py

@@ -142,7 +142,7 @@ class AnchorsTestCase(RuleTestCase):
                    problem4=(12, 3),
                    problem5=(13, 3),
                    problem6=(24, 7),
-                   problem7=(27, 36))
+                   problem7=(27, 37))
 
     def test_forbid_duplicated_anchors(self):
         conf = ('anchors:\n'