Use either of `eventHandler` and `defineEventHandler` consistently in documentation

[Nozomu Ikuta]

Describe the change

It is said in event-handler page that,

You can use eventHandler and defineEventHandler interchangeably. They are aliases. You can use the one you prefer but stick to it for consistency.

In this point, h3 docs doesn't stick to either of them.
Although documentation and its sample code are not actual application code,
it might be better to follow the convention that it tells by itself.

If @pi0 and/or @Barbapapazes tell me which to go, I will submit a PR to unify usages,
or please close this issue if it's fine to use both for some reasons.

URLs

All pages where eventHandler and defineEventHandler are used.

Additional information

• Would you be willing to help?

[Estéban]

You are right! 😄

You can update eventHandler to defineEventHandler which is I think the most used in the docs!