Various hyper-schema example fixes

[handrews]

What is it about publishing/releasing something that makes
errors jump out of text that you (and others) went over so
many times during final review?

• "base" URIs need to end in "/" for the "href" values to work,
  although when "base" has no path component it probably works
  either way
• "-wip" got left in the example meta-schema URIs :-(
• A cref about "-wip" being a placeholder got left in, although
  not anywhere near the examples where the "-wip" was otherwise
  apparent
• Tree node example lacked "$id", "$schema", and opening {
• There was no text in between the collection schema and instance

These are small but potentially quite confusing. I plan to add an errata
list to the web site, but after some time for other obvious errors to be
spotted it might be worth publishing a purely bugfix update as
draft-handrews-json-schema-hyperschema-01 (still with draft-07 meta-schema).

Apparently I did remember to remove all of the "-wip" from the validation spec.

[handrews]

I think what I'm going to do is merge PRs like this to both the master and draft-07 branches. Then if we decide that a bugfix draft is needed (meaning no specification changes, only corrections to align with the specification intended to be described by the draft-07 meta-schemas), we can publish that off of the draft-07 branch.

This would not be all that unusual btw. The reason that you don't see a draft-zyp-json-schema-00 on the IETF tracker chart is that it had an error and draft-zyp-json-schema-01 was published later that same day.