Rename count to length (#3166)

* update docs to refer to length instead of count

* rename count to length

* change all occurrences of 'count' to 'length' in tests

* format length command

crates/nu-command/tests/commands/group_by.rs

@@ -21,7 +21,7 @@ fn groups() {
                 open los_tres_caballeros.csv
                 | group-by rusty_at
                 | get "10/11/2013"
-                | count
+                | length
             "#
         ));