package/opencv3: fix typo in OPENCV_WARNINGS_ARE_ERRORS

OPENCV3_WARNINGS_ARE_ERRORS option does not exist so rename it to
OPENCV_WARNINGS_ARE_ERRORS (even if it is already disabled by default)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
1 file changed