docs: Remove outdated AllowPUTAsCreateMixin reference (#9788)

Author: zankoAn

docs/api-guide/generic-views.md

@@ -374,8 +374,6 @@ Allowing `PUT` as create operations is problematic, as it necessarily exposes in
 
 Both styles "`PUT` as 404" and "`PUT` as create" can be valid in different circumstances, but from version 3.0 onwards we now use 404 behavior as the default, due to it being simpler and more obvious.
 
-If you need to generic PUT-as-create behavior you may want to include something like [this `AllowPUTAsCreateMixin` class](https://gist.github.com/tomchristie/a2ace4577eff2c603b1b) as a mixin to your views.
-
 ---
 
 # Third party packages