chore: update ntex-cors doc to use http::Method instead of &str

[Rayato159]

This PR improves the documentation for allowed_methods in ntex-cors.

The current doc example uses vector of &str (e.g. vec!["GET", "POST"]), which is valid but might lead to runtime errors due to typos or unsupported values.

This update suggests using http::Method enum instead, which improves type safety and better aligns with idiomatic Rust practices.

Why?

• Prevent possible runtime errors
• Improve developer experience
• Promote usage of enums over raw strings

[fafhrd91]

thanks!