trivial cleanup for DOCS-11761: 4.2 remove group ref in explain page

Author: Kay Kim

source/reference/method/db.collection.explain.txt

@@ -25,7 +25,7 @@ Description
    :method:`~db.collection.aggregate()`;
    :method:`~db.collection.count()`;
    :method:`~db.collection.distinct()`;
-   :method:`~db.collection.find()`; :method:`~db.collection.group()`;
+   :method:`~db.collection.find()`;
    :method:`~db.collection.remove()`; and
    :method:`~db.collection.update()` methods.