Document the syntax for multiple redactions

[glennpratt]

I could not figure out how to have multiple redactions:

        assert_json_snapshot_matches!(
            "oci_json",
            &actual,
            { ".process.user.uid" => 42, ".process.user.gid" => 42, }
        );

error: expected expression, found `,`
   --> src/container/contained_v2.rs:407:14
    |
407 |             &actual,
    |              ^ expected expression

error: aborting due to previous error

If possible, could that be added to the README?

[glennpratt]

The thing I was missing was the trailing comma. Which I should have known because I've done that macro exercise somewhere along the way.