Tweaks to Quantity tutorial

This PR makes a few minor changes to the Quantity tutorial: First, while testing adrn/astropy-tutorials#6 , I noticed one of the links wasn't working... Apparently nbconvert doesn't automatically make "http://..." text into a link, so I just did it explicitly (and changed it to a text link).  

More significantly, it also changes the title.  I think this is just a slightly more "catchy" way of describing it, but if others (esp. @abonaca as the orginal author) prefer the original title, that's fine too and I can remove that part.

Note, @adrn, this conflicts slightly with adrn/astropy-tutorials#6 because it alters the metadata for the tutorial. It's easy to fix, but just worth noting because whichever one is merged second will need a rebase.