Fix the section reference in ch_ops_maintenance

The last line of "Total Compute Node Failure" section in "Maintenance,
Failures, and Debugging" chapter refers to "After a compute node Reboots"
for the detail of volume re-attaching. However, volume re-attaching is
described in "Volumes" section instead of "After a compute node Reboots".

This commit also fixes typo in ch_arch_network_design.xml.

Change-Id: Icc225369f9f7c9f41da74766478e40b3cf984891