Fixes path to minified code within Readme.

[sirbrad]

Hey @philipwalton.

Just a small fix to update the link url within the Usage section.

[philipwalton]

@sirbrad, sorry, I had to force push to the repo because we discovered some commits were missing from the history. Can you submit this again?

[roderickhsiao]

@philipwalton I guess the force push happens again so the readme now is reverted on master