[book] typos / clarifications in create-instance section
Clarify that cubicweb-ctl upgrade needs to be run whenever cubicweb or a
cube is upgraded. Related to #2632425.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/