minor: remove errant quotes

Author: salty-horse

source/reference/aggregation/sum.txt

@@ -12,5 +12,5 @@ $sum (aggregation)
    Alternately, if you specify a value as an argument,
    :group:`$sum` will increment this field by the specified value
    for every document in the grouping. Typically, as in the first
-   use above, specify a value of ``1`` " in order to count members of the
+   use above, specify a value of ``1`` in order to count members of the
    group.