[MINOR][DOC] Fix cursor parameter documentation for completion endpoint

Author: idzikovsky

docs/rest-api.md

@@ -362,7 +362,7 @@ Returns code completion candidates for the specified code in the session.
   <tr>
     <td>cursor</td>
     <td>cursor position to get proposals</td>
-    <td>string</td>
+    <td>int</td>
   </tr>
 </table>