Running tests docs should reference wptrun, not wptrunner directly

[RByers]

the running docs say to use wptrunner but on IRC jgraham said:

So wptrunner is the generic test running tool. wptrun is a frontend on that which sets it up correctly for running from a wpt checkout
Eventually wptrun will be spelled |wpt run| and there will be a bunch of other wpt subcommands
Yeah, using wptrunner directly is not really recommended since it's rather generic and hard to get going

The GitHub README says just to use wptrun and that seems much simpler and easier to setup. It's unfortunate that our two sources of docs can get out of date here. Since the web-platform-tests.org docs don't currently add any additional information on running, perhaps they should just link to the README for that to prevent this sort of confusion in the future?

[gsnedders]

This got fixed in #6751.