CMP0022: Fix version documented to support LINK_PUBLIC/LINK_PRIVATE

These have been available since 2.8.7, not 2.8.9 as the docs said.

Help/policy/CMP0022.rst

@@ -22,7 +22,7 @@ downstream users of CMake versions older than 2.8.12.  The
 target_link_libraries command will no longer populate the properties
 matching LINK_INTERFACE_LIBRARIES(_<CONFIG>)? if this policy is NEW.
 
-Warning-free future-compatible code which works with CMake 2.8.9 onwards
+Warning-free future-compatible code which works with CMake 2.8.7 onwards
 can be written by using the ``LINK_PRIVATE`` and ``LINK_PUBLIC`` keywords
 of :command:`target_link_libraries`.