Renamed time units (#2356)

* Changed time units as outlined in issue #2353.
Also applied changes to to_str for Unit - not sure if that was what was wanted.

* Forgot the tests!

* Updated primitive.rs to match changes.

* Updated where example to match changes.

* And the html test!

crates/nu-cli/src/commands/where_.rs

@@ -57,7 +57,7 @@ impl WholeStreamCommand for Where {
             },
             Example {
                 description: "List all files that were modified in the last two months",
-                example: "ls | where modified <= 2M",
+                example: "ls | where modified <= 2mon",
                 result: None,
             },
         ]