doc: fix the value type of circle-radius

[AmyFoxFN]

Hi,

Really thanks for the nice component suite. It's quite useful and graceful~

However, when I refer to the example of Adding Custom Data. I got a little problem. And find this little bug in example code:

The value of circle-radius should be a number.

And when setting as a string, it will break as throw an error like this:

Error: layers[0].paint.circle-radius: number expected, string found
    at Object.Jr [as emitValidationErrors] (webpack-internal:///./node_modules/mapbox-gl/dist/mapbox-gl.js:6005)
    at De (webpack-internal:///./node_modules/mapbox-gl/dist/mapbox-gl.js:18388)
    at i._load (webpack-internal:///./node_modules/mapbox-gl/dist/mapbox-gl.js:18441)
    at eval (webpack-internal:///./node_modules/mapbox-gl/dist/mapbox-gl.js:18436)

[CLAassistant]

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[Pessimistress]

Thank you!