DOCS-14819 fix to search_meta variable (#144)

* DOCS-14819 fix to search_meta variable

* Update source/reference/aggregation-variables.txt

Co-authored-by: jeff-allen-mongo <[email protected]>

Co-authored-by: jeff-allen-mongo <[email protected]>

Author: kanchana-mongodb

source/reference/aggregation-variables.txt

@@ -112,7 +112,7 @@ MongoDB offers the following system variables:
      - A variable that stores the metadata results of an :atlas:`Atlas 
        Search </atlas-search/>` query. In all supported aggregation 
        pipeline stages, a field set to the variable 
-       :variable:`$$SEARCH_META` returns the :atlas:`metadata results 
+       :variable:`$$SEARCH_META <SEARCH_META>` returns the :atlas:`metadata results 
        </reference/atlas-search/query-syntax/#metadata-result-types>`
        for the query.