asm-generic/page.h: fix typo in #error text requiring a real asm/page.h

Replace "need to prove a real asm/page.h" with "need to provide a real
asm/page.h"

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>