Fix link in installation doc
Fix link in installation doc

The link to building an image is not being properly parsed by sphinx in
the install user guide.

Change-Id: I1e73a4722b603ba11805031c522b107cae8c86b4

CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder