docs: fix Manager parent class in README example

[jasonfigueroa]

Fixes #401.

The README's Manager example does not compile when copied alongside the EmployeeType example. It references ManagerType, a private nested class, calls BonusSize as a method, declares the bonus as string instead of decimal, and assigns to an undefined _bonusSize_ field.

Use the public EmployeeType for the stored value and TryFromName lookup. Initialize the value to EmployeeType.Manager so callers can read the properties immediately. Expose BonusSize as a read-only decimal property.

Verification used a standalone .NET 10 console project that compiled the README snippets against the local SmartEnum source. Successive builds reproduced these compiler errors:

• CS0246 for the unresolved ManagerType reference.
• CS0122 after qualifying the private type as EmployeeType.ManagerType.
• CS1955 for calling BonusSize as a method.
• CS0029 for returning decimal through a string property.
• CS0103 for the undefined _bonusSize_ field.

The corrected example compiled and passed runtime checks. A new instance returned Manager with a bonus of 10000. Setting Type to "assistant" resolved Assistant with a bonus of 1000. An unknown name threw the expected exception and preserved the previous selection.