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

The https://templ.guide/syntax-and-usage/attributes/#json-attributes documentation states to create a function, but https://github.com/a-h/templ/blob/04abee5364c6fab2bde8c00d215fdcb630ad6a94/jsonstring.go#L7-L14 was designed for this purpose.

Need to update the docs.