Correct docs to reference the new libc++ lists.

We recently added libcxx-dev and libcxx-commits mailing lists.
This patch updates the libc++ documentation to correctly reference
the libc++ lists instead of the old Clang ones.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@342816 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed