Fix k#gen

The language about how to refer to the `gen` keyword in older editions
was changed in a recent commit from `k#gen` to `r#gen`.  This was
probably a search/replace error.  Let's fix that.

Author: traviscross

text/0000-gen-fn.md

@@ -85,7 +85,7 @@ Iterators created with `gen` return `None` once they `return` (implicitly at the
 
 Starting in the 2024 edition, `gen` is a keyword that cannot be used for naming any items or bindings.
 This means during the migration to the 2024 edition, all variables, functions, modules, types, etc. named `gen` must be renamed
-or be referred to via `r#gen`.
+or be referred to via `k#gen`.
 
 ## Returning/finishing an iterator