fix radius of the earth to reflect constants in code

Please see:

https://github.com/mongodb/mongo/pull/588

The code uses 6378.1 km as the radius of the earth. (3963.17 in miles)

https://github.com/mongodb/mongo/blob/master/src/mongo/db/geo/geoconstants.h

Could you update the documentation to reflect this value? I've found the following instances where the inconsistent value of 6371 km (3959 miles) is referenced.

http://docs.mongodb.org/manual/tutorial/calculate-distances-using-spherical-geometry-with-2d-geospatial-indexes/

http://docs.mongodb.org/manual/reference/operator/query/centerSphere/

http://docs.mongodb.org/manual/tutorial/query-a-2d-index/

http://docs.mongodb.org/manual/tutorial/query-a-2dsphere-index/