Fix intermittent failure on TestNato#test_codify

There were some cases where this test failed:

```bash
[274/375] TestNato#test_codify = 0.00 s
  1) Failure:
TestNato#test_codify [/Users/mateus/Code/ffaker/test/test_nato_alphabet.rb:28]:
Expected /[A-Z]+-[A-Z]+-[A-Z]+/ to match "SIERRA--VICTOR".
```

```bash
[274/375] TestNato#test_codify = 0.00 s
  1) Failure:
TestNato#test_codify [/Users/mateus/Code/ffaker/test/test_nato_alphabet.rb:28]:
Expected /[A-Z]+-[A-Z]+-[A-Z]+/ to match "MIKE--WHISKEY".
```

So I changed the regexp to make the "middle name" optional. I'm not sure if this is correct, but the test shouldn't be failing intermittently.