Correct duplicate TIFF tags

There was a typo in the spec, `BestQualityScale` should be 50780. 

http://www.asmail.be/msg0055321032.html 

Here's a corrected spec PDF, version 1.4.0.0 from June 2012:

https://a248.e.akamai.net/f/1953/8974/2h/wwwimages.adobe.com/content/dam/Adobe/en/products/photoshop/pdfs/dng_spec_1.4.0.0.pdf