Merge pull request #523 from AxiosCros/master

Fix spelling

Author: bast

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()