Redundant of removed.

Author: Karsten7

source/reference/operator/aggregation/toString.txt

@@ -17,7 +17,7 @@ Definition
 
    .. versionadded:: 4.0
 
-   Converts a value of to a string. If the value cannot be converted
+   Converts a value to a string. If the value cannot be converted
    to a string, :expression:`$toString` errors. If the value is null or
    missing, :expression:`$toString` returns null.