[polari] userList: Fix typo

[Gitlab Administrative User <gitlab src gnome org>]

commit d64d1ed5a41f083f49a2bdec647ff95bb576028f
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Nov 10 23:04:34 2017 +0100

    userList: Fix typo
    
    Whoops, this sneaked into commit caa067abcce342067 ...

 src/userList.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/userList.js b/src/userList.js
index afe9ca4..a198ceb 100644
--- a/src/userList.js
+++ b/src/userList.js
@@ -615,7 +615,7 @@ class UserList extends Gtk.ScrolledWindow {
     }
 
     get numRows() {
-        return this.rows.size;
+        return this._rows.size;
     }
 
     _onDestroy() {