docs(aggregation): Fix deprecated count(), add missing comma in , clarify sample data note

Author: ayaarragab

source/aggregation.txt

@@ -63,6 +63,9 @@ Aggregation Pipelines
 
 Aggregation Pipeline Example
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. note::
+   To run this example, load the ``sample_training`` dataset in MongoDB Atlas.
+   See :ref:`how to load sample data <load-sample-data>`.
 
 The following example pipeline uses documents from the :ref:`sample data 
 <sample-data>` available in MongoDB Atlas, specifically the 
@@ -140,7 +143,7 @@ The full pipeline resembles the following:
          $group : { 
             _id : { 
                "airline name": "$airline.name", 
-            }
+            },
             count : { 
                $sum: 1
             }
@@ -186,9 +189,10 @@ aggregation pipeline.
      - Returns an approximate count of the documents in a collection or
        a view.
 
-   * - :method:`db.collection.count()`
-     - Returns a count of the number of documents in a collection or a
-       view.
+   * - :method:`db.collection.countDocuments()`
+      - Returns an exact count of documents matching optional criteria.
+   * - :method:`db.collection.estimatedDocumentCount()`
+      - Returns a fast approximate count (no query filter support).
 
    * - :method:`db.collection.distinct()`
      - Returns an array of documents that have distinct values for the