fix: correctly compress sitemap.xml (#188)

Author: dkarlovi

src/Generator.php

@@ -167,7 +167,7 @@ private function dumpSitemap(Sitemapindex|Urlset $sitemap, bool $deflate): void
         }
 
         /** @var string $content */
-        $content = $deflate ? gzdeflate($sitemap->toXml()) : $sitemap->toXml();
+        $content = ($deflate ? gzencode($sitemap->toXml()) : $sitemap->toXml());
         $this->filesystem->dumpFile($this->buildDir.str_replace($this->generateUrl(''), '', $path), $content);
     }
 }