Fixes error in README

In the example for indentifying songs:

```
song = Echonest::Song.new('YOUR-API-KEY')
code = song.echoprint_code('path/to/audio/file')
puts code.identify(code)

```
`puts code.indentify(code)` should be `puts song.identify(code)` as
`code` is a string.
Merge pull request #19 from iMerica/README-Fix

Fixes error in README