Fixes to core spec

* Fix typo in URI resolution section.
* Change the wording about non JSON Pointer fragments in JSON References.
* Fix some spelling mistakes.

Thanks-to: Alain Fagot <alain.fagot@gmail.com>
Thanks-to: Tim Olson <kashapa@gmail.com>
Changes to the validation spec

* Suggested grammar fixes.
* Implementations should allow to disable "format" validation.

Thanks-to: Alain Fagot <alain.fagot.bearez@gmail.com>