fix: typo in docs (#199)

Author: danielgnh

README.md

@@ -295,7 +295,7 @@ User::orderByPowerJoins('profile.city');
 If you need to pass some raw values for the order by function, you can do like this:
 
 ```php
-User::orderByPowerJoins(['profile', DB::raw('concat(city, ", ", state)']);
+User::orderByPowerJoins(['profile', DB::raw('concat(city, ", ", state)')]);
 ```
 
 This query will sort the results based on the `city` column on the `user_profiles` table. You can also sort your results by aggregations (`COUNT`, `SUM`, `AVG`, `MIN` or `MAX`).