Update enums.md docs to reflect enum argument order when `raise_on_invalid_values` is set

[morriswchris]

In reading the doc additions for PR 164 (Manageable raising on invalid enum value) the example would throw an ArgumentError with

`enum': wrong number of arguments (given 0, expected 1..2) (ArgumentError)

This was due to how enum was being called in the example. This PR updates the docs the reflect the right method arguments.