Fix swapped closure parameters (nushell#1974)

As per [`reduce` command documentation examples][reduce], the reduce closure has parameters `|it, acc|`.

This accidental swap was part of the introduction of the example in nushell#1928.

[reduce]: https://www.nushell.sh/commands/docs/reduce.html

Author: Kissaki

lang-guide/chapters/types/basic_types/closure.md

@@ -177,7 +177,7 @@ The `reduce` command processes a list to accumulate a single result. The closure
 [1 2 3 4] | reduce { |accumulator, current_value| $accumulator + $current_value }
 ```
 
-_Explanation:_ This sums the numbers in the list. The closure `{|accumulator, current_value| $accumulator + $current_value}` adds the `current_value` to the `accumulator`. By default, the first item is the initial accumulator, and iteration starts from the second.
+_Explanation:_ This sums the numbers in the list. The closure `{|current_value, accumulator| $accumulator + $current_value}` adds the `current_value` to the `accumulator`. By default, the first item is the initial accumulator, and iteration starts from the second.
 
 **Output:**