fix convert ref (#222)

Author: jeff-allen-mongo

source/release-notes/5.2.txt

@@ -48,8 +48,8 @@ MongoDB 5.2 introduces the following aggregation operators:
 General Aggregation Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-:expression:`convert` Supports Timestamp Conversion to Date
-```````````````````````````````````````````````````````````
+:expression:`$convert` Supports Timestamp Conversion to Date
+````````````````````````````````````````````````````````````
 
 Starting in MongoDB 5.2, you can convert timestamps to dates using the 
 :expression:`$convert` operator. The :expression:`$toDate` operator can