[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

docs/DesignDocs/VisibilityMacros.rst

@@ -49,7 +49,7 @@ Visibility Macros
   attribute. With GCC the `visibility(...)` attribute is used and member
   functions are affected.
 
-**_LIBCPP_TEMPLATE_ONLY**
+**_LIBCPP_TEMPLATE_VIS**
   The same as `_LIBCPP_TYPE_VIS` except that it may be applied to class
   templates.