[C++] Fix the latest Boost version

Boost 1.68.0 isn't released yet.

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -146,7 +146,7 @@ set(Boost_ADDITIONAL_VERSIONS
   "1.62.0" "1.61"
   "1.61.0" "1.62"
   "1.60.0" "1.60")
-list(GET Boost_ADDITIONAL_VERSIONS 0 BOOST_LATEST_VERSION)
+list(GET Boost_ADDITIONAL_VERSIONS 2 BOOST_LATEST_VERSION)
 string(REPLACE "." "_" BOOST_LATEST_VERSION_IN_PATH ${BOOST_LATEST_VERSION})
 set(BOOST_LATEST_URL
   "https://dl.bintray.com/boostorg/release/${BOOST_LATEST_VERSION}/source/boost_${BOOST_LATEST_VERSION_IN_PATH}.tar.gz")