[plans] Use \placeholder in namespace

Author: jwakely

src/plans.tex

@@ -19,7 +19,7 @@
 When an extension defined in this or a future version of this technical
 specification represents enough existing practice, it will be moved into
 the next version of the \Cpp Standard by replacing the
-\tcode{experimental::net::vN} segment of its namespace with \tcode{net},
+\tcode{experimental::net::v\placeholder{N}} segment of its namespace with \tcode{net},
 and by removing the \tcode{experimental/} prefix from its header's path.