fixed typo. "collects" should be "collections" i think

Author: matulef

source/applications/aggregation.txt

@@ -63,7 +63,7 @@ through the pipeline.
 
 .. warning::
 
-   The pipeline cannot operate on collects with documents that contain
+   The pipeline cannot operate on collections with documents that contain
    one of the following "special fields:" ``MinKey``, ``MaxKey``,
    ``EOO``, ``Undefined``, ``DBRef``, ``Code``.