Since commit 39d334faa5 (package/pkg-qmake: add <pkg>_SYNC_QT_HEADERS
support), the qmake-package infra recognises said variable but the
manual has the wrong variable name, which is missing the "_QT" part.
We fix that by amending the manual to document the proper variable name.
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
ac2db5eb2e