Generic/UselessOverridingMethod: fix syntax errors in the documentation

This commit fixes two syntax errors in the PHP code examples in the
sniff documentation.

Previously, the provided code would cause the following PHP error:

```
PHP Fatal error: Cannot use "parent" when current class scope has no
parent
```

Author: rodrigoprimo

src/Standards/Generic/Docs/CodeAnalysis/UselessOverridingMethodStandard.xml

@@ -7,7 +7,7 @@
     <code_comparison>
         <code title="Valid: A method that extends functionality on a parent method.">
         <![CDATA[
-final class Foo
+final class Foo extends Baz
 {
     public function bar()
     {
@@ -19,7 +19,7 @@ final class Foo
         </code>
         <code title="Invalid: An overriding method that only calls the parent.">
         <![CDATA[
-final class Foo
+final class Foo extends Baz
 {
     public function bar()
     {