docs: fix typo in cmake-package documentation

The cmake-package documentation was referring to
BR2_PREFER_STATIC_LIBS, while the option is actually named
BR2_PREFER_STATIC_LIB. This commit fixes this mistake.

Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>