[libc++] Correct macro name in documenation

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

llvm-svn: 291330
diff --git a/libcxx/docs/DesignDocs/VisibilityMacros.rst b/libcxx/docs/DesignDocs/VisibilityMacros.rst
index 37539ec..7c6f39c 100644
--- a/libcxx/docs/DesignDocs/VisibilityMacros.rst
+++ b/libcxx/docs/DesignDocs/VisibilityMacros.rst
@@ -49,7 +49,7 @@
   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.