Update README to use proper plugin name.

Trying to apply the plugin 'com.jakewharton.hugo,' as specified in the README, failed for me. Taking a look at [u2020's build file](https://github.com/JakeWharton/u2020/blob/439bbb3a64526d2d08220f62b310547e3b143365/build.gradle#L20) and the [example's build file](https://github.com/JakeWharton/hugo/blob/489c9d125563ac5435ef19fa06317be14b810158/hugo-example/build.gradle#L25), found it used at 'hugo,' which worked :smile: 

Thanks for all the amazing projects!