fix searchMeta reference (#149)

Author: jeff-allen-mongo

source/reference/operator/aggregation-pipeline.txt

@@ -262,7 +262,7 @@ Alphabetical Listing of Stages
        To learn more, see :atlas:`Atlas Search Aggregation Pipeline 
        Stages </reference/atlas-search/query-syntax>`.
 
-   * - :pipeline:`$searchMeta </reference/operator/aggregation/searchMeta>`
+   * - :doc:`$searchMeta </reference/operator/aggregation/searchMeta>`
 
      - Returns different types of :ref:`metadata result 
        <fts-metadata-results>` documents for the :atlas:`Atlas Search