Update README to have 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)
fixed it right up :)