DATAMONGO-1662 - Fix classname in reference docs about projections in aggregations.

Original pull request: spring-projects#455.

Author: michael-simons

src/main/asciidoc/reference/mongodb.adoc

@@ -1704,7 +1704,7 @@ Note that the aggregation operations not listed here are currently not supported
 [[mongo.aggregation.projection]]
 === Projection Expressions
 
-Projection expressions are used to define the fields that are the outcome of a particular aggregation step. Projection expressions can be defined via the `project` method of the `Aggregate` class either by passing a list of ``String``'s or an aggregation framework `Fields` object. The projection can be extended with additional fields through a fluent API via the `and(String)` method and aliased via the `as(String)` method.
+Projection expressions are used to define the fields that are the outcome of a particular aggregation step. Projection expressions can be defined via the `project` method of the `Aggregation` class either by passing a list of ``String``'s or an aggregation framework `Fields` object. The projection can be extended with additional fields through a fluent API via the `and(String)` method and aliased via the `as(String)` method.
 Note that one can also define fields with aliases via the static factory method `Fields.field` of the aggregation framework that can then be used to construct a new `Fields` instance. References to projected fields in later aggregation stages are only valid by using the field name of included fields or their alias of aliased or newly defined fields. Fields not included in the projection cannot be referenced in later aggregation stages.
 
 .Projection expression examples