sort: use proper mass unit in example
sort: use proper mass unit in example

The values for the planet masses are given in
earth mass, not solar mass.

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/7368054

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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