String "Single Line" does not require double quotes; use single quotes instead

Author: Paul Bearne

tests/phpunit/tests/admin/includes/misc/insertWithMarkers.php

@@ -103,7 +103,7 @@ public function data_insert_with_markers() {
 			'String insertion instead of array'          => array(
 				'initial_content' => '',
 				'marker'          => 'StringTest',
-				'insertion'       => "Single Line",
+				'insertion'       => 'Single Line',
 				'expected'        => "\n# BEGIN StringTest\n{$string_desc}\nSingle Line\n# END StringTest",
 			),
 		);