link to the spec where it’s mentioned in the legend

I reworded that legend entry so that “spec” is its own word:

![after rewording](https://f.cloud.github.com/assets/79168/1097419/220edfc4-1711-11e3-8428-ccbac6732f10.png)

because it looked a little weird to have part of a word boldened like this:

![part of word boldened, without rewording](https://f.cloud.github.com/assets/79168/1097402/fe340214-1710-11e3-81f1-bce216fc45a1.png)

There is one other link to the spec on the whole page, in the “Getting Involved” section at the very bottom right.