Make the example in CONTRIBUTING imperative and concrete

Without this patch applied the example commit message in the CONTRIBUTING
document is not a concrete example. This is a problem because the
contributor is left to imagine what the commit message should look like
based on a description rather than an example. This patch fixes the
problem by making the example concrete and imperative.

The first line is a real life imperative statement with a ticket number
from our issue tracker. The body describes the behavior without the patch,
why this is a problem, and how the patch fixes the problem when applied.