Signature label should be a string, not array

[stainlesspot]

Hello, I'm coming from neoclide/coc.nvim#3385, where it was determined that racket-langserver for textDocument/signatureHelp returns label as an (singleton) array, instead of a string as the spec requires.

[Trace - 8:21:53 PM] Received response 'textDocument/signatureHelp - (2)' in 26ms.
Result: {
    "signatures": [
        {
            "documentation": "  char : char?",
            "label": [
                "(string char ...) -> string?"
            ]
        }
    ]
}

https://microsoft.github.io/language-server-protocol/specifications/specification-3-17/#signatureInformation