[Data] - Fix mongo datasource collStats invocation (ray-project#57027)

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## Why are these changes needed?

The `collStats` command had the wrong capitalization for s, so the
command failed on invocation.

https://www.mongodb.com/docs/manual/reference/command/collstats/

## Related issue number

<!-- For example: "Closes ray-project#1234" -->

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Fixes MongoDB command name to `collStats` when retrieving `avgObjSize`
in `MongoDatasource`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
b017c04. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Signed-off-by: Goutam V. <goutam@anyscale.com>

Author: goutamvenkat-anyscale

python/ray/data/_internal/datasource/mongo_datasource.py

@@ -52,7 +52,7 @@ def _get_or_create_client(self):
                 self._client, self._database, self._collection
             )
             self._avg_obj_size = self._client[self._database].command(
-                "collstats", self._collection
+                "collStats", self._collection
             )["avgObjSize"]
 
     def get_read_tasks(