exercises(secret-handshake): consider changing `Signal` to `Action`

[ee7]

The tests currently use Signal:

https://github.com/exercism/zig/blob/0c4805d47884d6ec790bc1cae43f13e9e9fadaeb/exercises/practice/secret-handshake/test_secret_handshake.zig#L7

But the docs only talk about "actions":

https://github.com/exercism/zig/blob/0c4805d47884d6ec790bc1cae43f13e9e9fadaeb/exercises/practice/secret-handshake/.docs/instructions.md#L3-L8