[BUGFIX] Use static prefix as documented

The documentation says:

    TCEMAIN.pages.slug.generatorOptions {
        prefix = /wishlist/
    }

but the code worked like this:

    TCEMAIN.pages.slug {
        prefix = /wishlist/
    }

The bugfix makes it consistent in the
"generatorOptions" namespace, as this
option is fully related for the
generation of a URL. This way,
the documentation does not need adaption.

THE CORRECT WAY NOW IS THIS:

    TCEMAIN.pages.slug.generatorOptions {
        prefix = /wishlist/
    }

Classes/SlugModifier.php

@@ -124,7 +124,7 @@ protected function resolveHookParameters(array $configuration, $tableName, $fiel
      */
     protected function regenerateSlug(SlugHelper $helper): string
     {
-        $prefix = $this->configuration['prefix'] ?? '';
+        $prefix = $this->configuration['generatorOptions']['prefix'] ?? '';
         if ($this->configuration['generatorOptions']['prefixParentPageSlug'] ?? false) {
             $languageFieldName = $GLOBALS['TCA'][$this->tableName]['ctrl']['languageField'] ?? null;
             $languageId = (int)($this->recordData[$languageFieldName] ?? 0);