[BUGFIX] Return type of content altering method set to string

Classes/Service/PlaintextService.php

@@ -97,9 +97,9 @@ class PlaintextService {
 	 * add linebreaks on some parts (</p> => </p><br>)
 	 *
 	 * @param string $content
-	 * @return array
+	 * @return string
 	 */
-	protected function addLineBreaks($content): array {
+	protected function addLineBreaks($content): string {
 		$tags2LineBreaks = [
 			'</p>',
 			'</tr>',