refactor: improve group example wording and formatting

Author: ricardohsmello

java/104_aggregation_pipeline_group_by.ipynb

@@ -46,6 +46,7 @@
     "import com.mongodb.client.model.Filters;\n",
     "\n",
     "import static com.mongodb.client.model.Sorts.descending;\n",
+    "import static com.mongodb.client.model.Aggregates.limit;\n",
     "\n",
     "import org.bson.Document;\n",
     "import java.util.Arrays;\n",
@@ -78,7 +79,7 @@
    "source": [
     "## $group\n",
     "\n",
-    "`$group` groups books by `year` and calculates the total number of pages for each group."
+    "Use `$group` to group books by `year` and calculate the total number of pages for each year and limit in 5 results."
    ]
   },
   {
@@ -89,8 +90,12 @@
    "outputs": [],
    "source": [
     "AggregateIterable<Document> result = books.aggregate(\n",
-    "    Arrays.asList(\n",
-    "        Aggregates.group(\"$year\", Accumulators.sum(\"totalPages\", \"$pages\"))\n",
+    "    List.of(\n",
+    "        Aggregates.group(\n",
+    "            \"$year\",\n",
+    "            Accumulators.sum(\"totalPages\", \"$pages\")\n",
+    "        ),\n",
+    "        limit(5)\n",
     "    )\n",
     ");\n",
     "\n",