Fix - Documented unit used for Earth's radius

[IvoBiaus]

Corrected the "unit" that is documented as the one being used for the Earth's radius, from "kilometers" to "meters".

The constant value is: 6371008.8, which is + 6 million, that would be meters.
As mentioned in the Wikipedia that is referenced in the documentation https://en.wikipedia.org/wiki/Earth_radius#Arithmetic_mean_radius

For Earth, the arithmetic mean radius is 6,371.0088 km (3,958.7613 mi).

6371 km are 6.371.000 meters.

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This change should not bring any errors, since only changes the documentation, instead of changing the constant value to kilometers.

I took a quick look, and the code seems to have been done using the correct "meters" unit, for example when checking out the factors constant:

export const factors: Record<Units, number> = {
  centimeters: earthRadius * 100,
  centimetres: earthRadius * 100,
  degrees: 360 / (2 * Math.PI),
  feet: earthRadius * 3.28084,
  inches: earthRadius * 39.37,
  kilometers: earthRadius / 1000,  // <------
  kilometres: earthRadius / 1000,  // <------
  meters: earthRadius,  // <------
  metres: earthRadius,  // <------
  miles: earthRadius / 1609.344,
  millimeters: earthRadius * 1000,
  millimetres: earthRadius * 1000,
  nauticalmiles: earthRadius / 1852,
  radians: 1,
  yards: earthRadius * 1.0936,
};

[smallsaucepan]

Thanks for fixing this @IvoBiaus 👍