Fix a typo in an assertion

[kenranunderscore]

Small chance that it's by design, but I think it's a typo: Not matching [...] -> No matching.

  1) test immediately refreshes expired tokens (Twitch.AuthTest)
     test/twitch/auth_test.exs:21
     Not matching function found for Elixir.Twitch.Auth.Effect: refresh_token/3
     code: bind(Effect, :refresh_token, fn _, _, _ ->
     stacktrace:
       (efx 0.2.11) lib/efx_case/mock_state.ex:57: EfxCase.MockState.add_fun/6
       test/twitch/auth_test.exs:34: (test)