Use ~c to represent charlist arg for :gen_tcp.connect/3

[bigbluejay9]

Suppresses compiler warning about single quoted strings.

Before:

    warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
    │
 12 │     {:ok, socket} = :gen_tcp.connect('localhost', 4040, opts)
    │                                      ~
    │
    └─ test/kv_server_test.exs:12:38

After:
no warning emitted.

[josevalim]

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