Installation documentation typo

[pbiolsi]

I was unable to resolve a dependency error (ruby error cannot load such file -- image-size in my build until changing my jekyll _config.yml file from:

plugins:
  - image-size

(per the documenation), to the following:

plugins:
  - jekyll-image-size

Not sure the cause of this since I installed via bundler and saw the dependency in my project Gemfile. Perhaps updating the doc will help others to avoid this.