docs/manual: fix typo in name of a qmake-package variable

Since commit 39d334fa (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>