Reviewed and tweaked wording of services document, no substantive changes

Have reviewed https://github.com/IIIF/iiif.io/blob/master/source/api/annex/services/index.md and made a number of tweaks. In particular I took a slightly less story-telling approach to the GeoJSON intro. Instead of saying "we don't want to open the door for you to do (bad things)" I put it as "here is what we intend. It is not appropriate to use this feature for (bad things)".