trivial: fix link to sum agg operator

Author: kay-kim

source/core/transactions.txt

@@ -98,11 +98,11 @@ Count Operation
 
 To perform a count operation within a transaction, use the
 :pipeline:`$count` aggregation stage or the :pipeline:`$group` (with a
-:expression:`$sum` expression) aggregation stage.
+:group:`$sum` expression) aggregation stage.
 
 MongoDB drivers compatible with the 4.0 features provide a
 collection-level API ``countDocuments(filter, options)`` as a helper
-method that uses the :pipeline:`$group` with a :expression:`$sum`
+method that uses the :pipeline:`$group` with a :group:`$sum`
 expression to perform a count.
 
 .. _transactions-ops-info: