[libc++] Fix typo in documentation for LIBCXX_HERMETIC_STATIC_LIBRARY

Thanks to Yichen Yan for the patch.
Differential Revision: https://reviews.llvm.org/D66675

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369800 91177308-0d34-0410-b5e6-96231b3b80d8