Update link to FactoryGirl upgrade guide (#1769)

Author: imRohan

README.md

@@ -9,7 +9,7 @@ If you want to use factory_bot with Rails, see
 
 Interested in the history of the project name? You can find the history [here](https://github.com/thoughtbot/factory_bot/blob/main/NAME.md)
 
-Transitioning from factory\_girl? Check out the [upgrade guide](https://github.com/thoughtbot/factory_bot/blob/4-9-0-stable/UPGRADE_FROM_FACTORY_GIRL.md).
+Transitioning from factory\_girl? Check out the [upgrade guide](https://github.com/thoughtbot/factory_bot/blob/v4.9.0/UPGRADE_FROM_FACTORY_GIRL.md).
 
 ## Documentation