Fix spelling

Fix spelling of `_bootstrap_select_libraries` variable

Signed-off-by: Axios <13862149+AxiosCros@users.noreply.github.com>

Author: AxiosLeo

chapter-08/recipe-02/cxx-example/external/upstream/boost/CMakeLists.txt

@@ -100,5 +100,5 @@ else()
   unset(_b2_needed_components)
   unset(_build_byproducts)
   unset(_b2_select_libraries)
-  unset(_boostrap_select_libraries)
+  unset(_bootstrap_select_libraries)
 endif()