Update error example

[antonlau-1]

When discussing best practices for Go's error packages, the current examples define static sentinel errors using fmt.Errorf().

I believe this is undesirable for two reasons:

• Misleading intent. Using fmt.Errorf() without format verbs suggests formatting is occurring when it is not, causing readers to look for format verbs that do not exist.
• Unnecessary dependency. fmt is a large formatting package. For truly static errors, there is no reason to import it when errors.New() is designed for this use case.

This change clarifies the separation of concerns: errors.New() for static error declarations,fmt.Errorf() when formatting is actually needed.

[antonlau-1]

Ran locally to check the displayed output.

[eliben]

Thanks