graphics/okular: fix PURPOSE option's CMAKE_ARGS

The cmake package was renamed a while ago, so this had been broken for a
while technically. However with the latest release the optional
dependencies have been made required by default, leading to it now
actually erroring out in this case.

Reported by:	Sebastian <sko@rostwald.de>
PR:		266637

graphics/okular/Makefile

@@ -41,6 +41,6 @@ OPTIONS_DEFAULT=	PURPOSE
 
 PURPOSE_DESC=	Enable 'Share' menu
 PURPOSE_USE=	KDE=purpose
-PURPOSE_CMAKE_BOOL_OFF=	CMAKE_DISABLE_FIND_PACKAGE_KDEExperimentalPurpose
+PURPOSE_CMAKE_BOOL_OFF=	CMAKE_DISABLE_FIND_PACKAGE_KF5Purpose
 
 .include <bsd.port.mk>