fix pe_ratio column

After attempting to use this gem in an app, it became clear that the P/E Ratio it was getting was incorrect.  I googled around to find Yahoo documentation on the available columns and was not able to find it on their site.  I did, however, find documentation on several other sites that say that pe_ratio should be column 'r0'.  After the change, the data I'm getting appears to be correct.

It's a simple one-line change - it would be great if you could merge it in.