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

Since commit 39d334faa5c1 (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>

docs/manual/adding-packages-qmake.txt

@@ -80,6 +80,6 @@ also be defined.
   to the +make+ command during the target installation step. By default,
   +install+.
 
-* +LIBFOO_SYNC_HEADERS+, to run syncqt.pl before qmake. Some packages
+* +LIBFOO_SYNC_QT_HEADERS+, to run syncqt.pl before qmake. Some packages
   need this to have a properly populated include directory before
   running the build.