docs(README.md): correct hook name in example and remove misleading comment

[kyselberg]

Summary

this pull request changes hook usage example

Background & Context

now hookEvent is passed only for uponSanitizeAttribute hook.
currently there is misleading example of hook usage, where the beforeSanitizeElements hook is used.

in order to avoid misunderstandings and simplify life for next users of lib, let's correct this mistake.

[cure53]

Oh, well spotted, thank you!