[libc++] Correct macro name in documenation

The macro is named `_LIBCPP_TEMPLATE_VIS`, not `_LIBCPP_TEMPLATE_ONLY`.
No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291330 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed