Use the term Living Standard, not Living Specification

https://wiki.whatwg.org/wiki/FAQ#What_does_.22Living_Standard.22_mean.3F
https://dom.spec.whatwg.org/
https://html.spec.whatwg.org/
https://url.spec.whatwg.org/
Merge pull request #132 from foolip/living-standard

Use the term Living Standard, not Living Specification