doc: add table header in intl.md

An empty header element in a table is an accessibility issue. In this
case, specifying the column as containing a feature seems to be the way
to go.

PR-URL: #36261
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/intl.md

@@ -42,7 +42,7 @@ in [BUILDING.md][].
 An overview of available Node.js and JavaScript features for each `configure`
 option:
 
-|                                         | `none`                            | `system-icu`                 | `small-icu`            | `full-icu` |
+| Feature                                 | `none`                            | `system-icu`                 | `small-icu`            | `full-icu` |
 |-----------------------------------------|-----------------------------------|------------------------------|------------------------|------------|
 | [`String.prototype.normalize()`][]      | none (function is no-op)          | full                         | full                   | full       |
 | `String.prototype.to*Case()`            | full                              | full                         | full                   | full       |