doc: add ver of 18.x where Node-api 9 is supported

Signed-off-by: Michael Dawson <mdawson@devrus.com>
PR-URL: nodejs#48876
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

Author: mhdawson

doc/api/n-api.md

@@ -259,7 +259,7 @@ information is in the latest API documentation in:
   </tr>
   <tr>
     <th scope="row">9</th>
-    <td> 20.3.0+, 21.0.0 and all later versions</td>
+    <td>v18.17.0+, 20.3.0+, 21.0.0 and all later versions</td>
   </tr>
   <tr>
     <th scope="row">8</th>