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.

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,
};