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>

docs/manual/adding-packages-cmake.txt

@@ -102,7 +102,7 @@ typical packages will therefore only use a few of them.
 
 ** +CMAKE_BUILD_TYPE+ is driven by +BR2_ENABLE_DEBUG+;
 ** +CMAKE_INSTALL_PREFIX+;
-** +BUILD_SHARED_LIBS+ is driven by +BR2_PREFER_STATIC_LIBS+;
+** +BUILD_SHARED_LIBS+ is driven by +BR2_PREFER_STATIC_LIB+;
 ** +BUILD_DOC+, +BUILD_DOCS+ are disabled;
 ** +BUILD_EXAMPLE+, +BUILD_EXAMPLES+ are disabled;
 ** +BUILD_TEST+, +BUILD_TESTS+, +BUILD_TESTING+ are disabled.