Updated README according to Express 4.0

`/html$/` is a regexp :)

So instead of parsing a whole string for replacing a subject, I simply cut off the last part of it by the length of the subject - `.slice(0, -'html'.length)`, and afterwards I concatenate the "replacement". In theory, it will work faster. In practice - telling the truth, I am not sure if it has any significant impact on the URL strings :)

Anyway, I just wanted to be thankful for your snippet :+1: