docs: update fmt section in config file manual

[ElGranTorino]

I refactored the config file for compatibility with the new Deno version. The changes include replacing the "fmt" section with the following structure:

"fmt": {
    "options": {
      "lineWidth": 80,
      "indentWidth": 4,
      "semiColons": false,
      "singleQuote": true,
      "proseWrap": "preserve"
    },
    "files": {
      "exclude": ["src/"],
      "include": ["src/testdata/", "data/fixtures/**/*.ts"]
    }
},

instead of

"fmt": {
    "useTabs": true,
    "lineWidth": 80,
    "indentWidth": 4,
    "semiColons": false,
    "singleQuote": true,
    "proseWrap": "preserve",
    "include": ["src/"],
    "exclude": ["src/testdata/", "data/fixtures/**/*.ts"]
},

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[kt3k]

I don't see the motivation of this change well. The existing example uses newer forms of configuration, which were introduced in 1.33 ( https://deno.com/blog/v1.33#flatter-denojson-configuration )