doc/configuration: Fix incorrect name "defaults"

See YamlParser#applyDefaults in builder.py.

Change-Id: I763d23608b79f7b1c560ae41cf5ca0d2dcc19d60
Reviewed-on: https://review.openstack.org/24407
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins