Update Mix.Config mentions (#13115)

ExDoc main emitted these warnings on Elixir main:

```
    warning: documentation references module "Mix.Config" but it is hidden
    │
 49 │   `Mix.Config`, which was specific to Mix and has been deprecated.
    │   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/elixir/lib/config.ex:49: Config (module)

    warning: documentation references module "Mix.Config" but it is hidden
    │
 51 │   You can leverage `Config` instead of `Mix.Config` in three steps. The first
    │   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/elixir/lib/config.ex:51: Config (module)
```

lib/elixir/lib/config.ex

@@ -46,9 +46,9 @@ defmodule Config do
   ## Migrating from `use Mix.Config`
 
   The `Config` module in Elixir was introduced in v1.9 as a replacement to
-  `Mix.Config`, which was specific to Mix and has been deprecated.
+  `use Mix.Config`, which was specific to Mix and has been deprecated.
 
-  You can leverage `Config` instead of `Mix.Config` in three steps. The first
+  You can leverage `Config` instead of `use Mix.Config` in three steps. The first
   step is to replace `use Mix.Config` at the top of your config files by
   `import Config`.