[FIX] web_timeline: Wrap fields_get arg into a list

By contract the first argument of the `fields_get` method is supposed to
be a list. Before this fix, `web_timeline` would call `fields_get` with
a string instead of a list.

Fortunately in case only 1 field is being grouped, this worked as Odoo
does an `x in y` comparison in its `fields_get` implementation, which
does pass in a simple `"project_id" in "project_id"` case.

But that call remains invalid and can break when `fields_get` has been
tweaked by other modules.

web_timeline/readme/CONTRIBUTORS.rst

@@ -10,6 +10,12 @@
   * Pedro M. Baeza
   * Alexandre Díaz
   * César A. Sánchez
+
 * `Onestein <https://www.onestein.nl>`_:
-    * Dennis Sluijk <d.sluijk@onestein.nl>
-    * Anjeel Haria
+
+  * Dennis Sluijk <d.sluijk@onestein.nl>
+  * Anjeel Haria
+
+* `XCG Consulting <https://xcg-consulting.fr>`_:
+
+  * Houzéfa Abbasbhay

web_timeline/static/src/js/timeline_renderer.js

@@ -399,7 +399,7 @@ odoo.define("web_timeline.TimelineRenderer", function (require) {
                             await this._rpc({
                                 model: this.modelName,
                                 method: "fields_get",
-                                args: [grouped_field],
+                                args: [[grouped_field]],
                                 context: this.getSession().user_context,
                             }).then(async (fields) => {
                                 if (fields[grouped_field].type === "many2many") {