update mario checks to look for trailing whitespace and advise accordingly

[dmalan]

It would be handy to mention on the guidelines that you don't need to add spaces on the other side of the right hand column to make the spaces and pound signs symmetrical (i.e., that it's ragged right).

[brianyu28]

We actually already do this, per:

https://github.com/cs50/checks/blob/master/cs50/2017/fall/mario/less/check50/__init__.py#L76

See the detailed link in the screenshot for what that looks like:

https://cs50.me/checks/e9a0cb104971aa13e0370849151e1660ee7bfaf9

[dmalan]

Ah, hm, so it's the abbreviated output that's referred to. Should we perhaps highlight spaces in the web output to make things more clear?