docs: fix suggestion to create your own JSON attribute function when not required

[a-h]

The https://templ.guide/syntax-and-usage/attributes/#json-attributes documentation states to create a function, but

templ/jsonstring.go

Lines 7 to 14 in 04abee5

	// JSONString returns a JSON encoded string of v.
	func JSONString(v any) (string, error) {
	b, err := json.Marshal(v)
	if err != nil {
	return "", err
	}
	return string(b), nil
	}

was designed for this purpose.

Need to update the docs.