Update README to match style guide

Thanks to @froschdesign for pointing this out in
mezzio#58 (comment).

README.md

@@ -70,7 +70,7 @@ The command supports several options, listed in the table below.
 
 By default, `Mezzio\Tooling\Routes\DefaultRoutesConfigLoaderFactory` registers a `ConfigLoaderInterface` service with the application's DI container, which retrieves the application's routes from two sources:
 
-- _config/routes.php_
+- `config/routes.php`
 - Routes registered by any loaded `ConfigProvider` class
 
 However, this is a default/fallback implementation.