Fix broken links in closure reference

Author: nome

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

@@ -122,14 +122,14 @@ Closures are used in Nu extensively as parameters to iteration style commands li
 - [`all`](/commands/docs/all.md)
 - [`any`](/commands/docs/any.md)
 - [`collect`](/commands/docs/collect.md)
-- [`do`](/commands/docs/do`.md)
+- [`do`](/commands/docs/do.md)
 - [`each`](/commands/docs/each.md)
 - [`explain`](/commands/docs/explain.md)
 - [`filter`](/commands/docs/filter.md)
-- [`group-by`](/commands/docs/group.md)
+- [`group-by`](/commands/docs/group-by.md)
 - [`interleave`](/commands/docs/interleave.md)
 - [`items`](/commands/docs/items.md)
-- [`par-each`](/commands/docs/par.md)
+- [`par-each`](/commands/docs/par-each.md)
 - [`reduce`](/commands/docs/reduce.md)
 - [`skip until`](/commands/docs/skip_until.md)
 - [`skip while`](/commands/docs/skip_while.md)