docs(error): simplify status code retrieval

[tongium]

Description

This small docs change updates the custom error handler example to use the existing echo.StatusCode(err) helper instead of the manual errors.As check.

Why:

1. It makes getting the error code simpler and easier to read.
2. Running go fix on the current snippet rewrites it into the generic form. Reusing echo.StatusCode(err) avoids that and keeps the example stable.

[tongium]

I think good to show how to works, it make user understand interface, so I decide to close PR.