minor: fixes to broken cross references

Author: Sam Kleinman

source/reference/aggregation/cond.txt

@@ -6,7 +6,7 @@ $cond (aggregation)
 
 .. expression:: $cond
 
-   Use the :operator:`$cond` operator with the following syntax:
+   Use the :expression:`$cond` operator with the following syntax:
 
    .. code-block:: javascript
 

source/reference/aggregation/ifNull.txt

@@ -6,7 +6,7 @@ $ifNull (aggregation)
 
 .. expression:: $ifNull
 
-   Use the :operator:`$ifNull` operator with the following syntax:
+   Use the :expression:`$ifNull` operator with the following syntax:
 
    .. code-block:: javascript