fix documentation: changes to htmldiff have to be updated in the docu

[hu3bi]

Recent changes to htmldiff and Snapshot testing should be represented in the documentation:

https://github.com/a-h/templ/blob/main/docs/docs/04-core-concepts/03-testing.md

The signature was changed from:

func Diff(input templ.Component, expected string) (diff string, err error)

to

func Diff(input templ.Component, expected string) (actual, diff string, err error)

Proposal:
Update the documentation to accommodate signature changes.

It now also needs the prettier command available. So it should be added, that prettier needs to be installed and in the path var of the shell.