[docs] Update IPA images urls

Starting with Zed, Ironic builds IPA on centos9.

Change-Id: I7bacd392944148f471ec421301bfc8106a55b68a

doc/source/reference/bare-metal/ironic-guide.rst

@@ -82,10 +82,10 @@ be used:
 
 .. code-block:: console
 
-   $ curl https://tarballs.opendev.org/openstack/ironic-python-agent/dib/files/ipa-centos8-|KOLLA_BRANCH_NAME_DASHED|.kernel \
+   $ curl https://tarballs.opendev.org/openstack/ironic-python-agent/dib/files/ipa-centos9-|KOLLA_BRANCH_NAME_DASHED|.kernel \
      -o /etc/kolla/config/ironic/ironic-agent.kernel
 
-   $ curl https://tarballs.opendev.org/openstack/ironic-python-agent/dib/files/ipa-centos8-|KOLLA_BRANCH_NAME_DASHED|.initramfs \
+   $ curl https://tarballs.opendev.org/openstack/ironic-python-agent/dib/files/ipa-centos9-|KOLLA_BRANCH_NAME_DASHED|.initramfs \
      -o /etc/kolla/config/ironic/ironic-agent.initramfs
 
 You may optionally pass extra kernel parameters to the inspection kernel using: