Clarify that an accessURL may be the same URL used for landingPage

[philipashlock]

This is called out in the Usage Note for landingPage in DCAT:

If the distribution(s) are accessible only through a landing page (i.e. direct download URLs are not known), then the landing page link SHOULD be duplicated as accessURL on a distribution. (see example 4.4)

Once instance where they might not be the same would be if there are multiple distributions for a dataset and each distribution had a different accessURL. In that case, you might have a landingPage URL that acts as the landing page for all distributions.