Update object-mapper.md link
Update object-mapper.md link

object-mapper.md is moving under architecture/ as part of
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/26250/

26250 should merge before this is merged.

Change-Id: Icf4ace8d827cd6fbfb61f41008b4662635755a3c
Signed-off-by: Gunnar Mills <213d3186dd7abf62ec3e856bb92d7b256149744c@us.ibm.com>

openbmc/phosphor-inventory-manager,openbmc/phosphor-inventory-manager