Help: Add OBJECT_LIBRARY to TYPE target property documentation

Help/prop_tgt/TYPE.rst

@@ -5,5 +5,5 @@ The type of the target.
 
 This read-only property can be used to test the type of the given
 target.  It will be one of STATIC_LIBRARY, MODULE_LIBRARY,
-SHARED_LIBRARY, INTERFACE_LIBRARY, EXECUTABLE or one of the internal
-target types.
+SHARED_LIBRARY, OBJECT_LIBRARY, INTERFACE_LIBRARY, EXECUTABLE or one
+of the internal target types.