Fix typo in test fixture

Author: jrfnl

tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.xml

@@ -19,6 +19,6 @@
     <!-- Breaks: No directories should exist under the [CategoryName] directory. -->
     <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/Category/SubDir/TooDeeplyNestedSniff.php"/>
 
-    <!-- Prevent a "no sniff were registered" error (once the above become errors). -->
+    <!-- Prevent a "no sniffs were registered" error (once the above become errors). -->
     <rule ref="Generic.PHP.BacktickOperator"/>
 </ruleset>