Typo: Delete the duplicate word to

Typo: Delete the duplicate word `to`

Author: Jeff-Tian

source/includes/extracts-agg-stages.yaml

@@ -13,7 +13,7 @@ content: |
         - Adds new fields to documents. Similar to
           :pipeline:`$project`, :pipeline:`$addFields` reshapes each
           document in the stream; specifically, by adding new fields to
-          to output documents that contain both the existing fields
+          output documents that contain both the existing fields
           from the input documents and the newly added fields.
 
       * - :pipeline:`$bucket`