fix typo "aggragegate"

Author: AlvesJorge

source/reference/aggregation.txt

@@ -103,7 +103,7 @@ are supported:
 To construct a pipeline, call the corresponding aggregation pipeline methods
 on a ``Criteria`` instance. Aggregation pipeline operations are added to the
 ``pipeline`` attribute of the ``Criteria`` instance. To execute the pipeline,
-pass the ``pipeline`` attribute value to ``Collection#aggragegate`` method.
+pass the ``pipeline`` attribute value to ``Collection#aggregate`` method.
 
 For example, given the following models: