Use ~c to represent charlist arg for connect/3 (#13837)

Suppresses compiler warning about single quoted strings.

```
warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
```

Author: bigbluejay9

lib/elixir/pages/mix-and-otp/docs-tests-and-with.md

@@ -371,7 +371,7 @@ defmodule KVServerTest do
 
   setup do
     opts = [:binary, packet: :line, active: false]
-    {:ok, socket} = :gen_tcp.connect('localhost', 4040, opts)
+    {:ok, socket} = :gen_tcp.connect(~c"localhost", 4040, opts)
     %{socket: socket}
   end