Fix linting issue

Author: michaelw85

src/Context/GivenStepDefinitions.php

@@ -184,7 +184,7 @@ public function given_a_request_to_a_url_respond_with_file( $url_or_pattern, PyS
 			mkdir( $dir, 0777, true /*recursive*/ );
 		}
 
-		$config_file_contents = <<<FILE
+		$config_file_contents = <<<'FILE'
 require:
     - mock-requests.php
 FILE;