doc/dev/config: fix verb disagreement

doc/dev/config: fix verb disagreement

This PR changes the following sentence:

   The other is to register an observer
   that will be called every time the
   relevant configuration values changes.

This sentence has been rewritten so that the
verb agrees with its subject, like this:

   The other is to register an observer
   that will be called every time the
   relevant configuration values change.

Signed-off-by: Zac Dover <zac.dover@gmail.com>