[files_external] Fix phpdoc

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

apps/files_external/lib/Lib/Backend/Backend.php

@@ -82,7 +82,7 @@ class Backend implements \JsonSerializable {
 
 	/**
 	 * @param string $class
-	 * @return self
+	 * @return $this
 	 */
 	public function setStorageClass($class) {
 		$this->storageClass = $class;

apps/files_external/lib/Lib/IdentifierTrait.php

@@ -63,7 +63,7 @@ trait IdentifierTrait {
 
 	/**
 	 * @param string $alias
-	 * @return self
+	 * @return $this
 	 */
 	public function addIdentifierAlias($alias) {
 		$this->identifierAliases[] = $alias;