fix(cmake): use the right variable exported from BoostConfig.cmake

Author: wravery

samples/CMakeLists.txt

@@ -15,7 +15,7 @@ if(GRAPHQL_BUILD_HTTP_SAMPLE)
       try_compile(TEST_RESULT
         ${CMAKE_CURRENT_BINARY_DIR}
         ${CMAKE_CURRENT_SOURCE_DIR}/../cmake/test_boost_beast.cpp
-        CMAKE_FLAGS -DINCLUDE_DIRECTORIES:STRING=${Boost_INCLUDE_DIR}
+        CMAKE_FLAGS -DINCLUDE_DIRECTORIES:STRING=${Boost_INCLUDE_DIRS}
         CXX_STANDARD 20)
 
       if(TEST_RESULT)