docs: update reference to main branch

Author: G-Rath

guides/source/i18n.md

@@ -312,7 +312,7 @@ private
 ```
 
 
-In practice, more robust code is necessary to do this reliably. Iain Hecker's [http_accept_language](https://github.com/iain/http_accept_language/tree/master) library or Ryan Tomayko's [locale](https://github.com/rack/rack-contrib/blob/master/lib/rack/contrib/locale.rb) Rack middleware provide solutions to this problem.
+In practice, more robust code is necessary to do this reliably. Iain Hecker's [http_accept_language](https://github.com/iain/http_accept_language/tree/master) library or Ryan Tomayko's [locale](https://github.com/rack/rack-contrib/blob/main/lib/rack/contrib/locale.rb) Rack middleware provide solutions to this problem.
 
 ##### Inferring the Locale from IP Geolocation