Redundant word removed.

Author: Karsten7

source/reference/operator/aggregation/toObjectId.txt

@@ -21,7 +21,7 @@ Definition
 
    .. versionadded:: 4.0
 
-   Converts a value of to an :method:`ObjectId`. If the value cannot be
+   Converts a value to an :method:`ObjectId`. If the value cannot be
    converted to an ObjectId, :expression:`$toObjectId` errors. If the
    value is null or missing, :expression:`$toObjectId` returns null.