Fixed typo about sectors 511 and 512

Statement "signature bytes `0x55` and `0xAA` at *sectors* 511 and 512 respectively." isn't correct. There should be "offsets" instead of "sectors", because signature belongs only to first (mbr) sector, not to 511th and 512th sectors.
Merge pull request #52 from stunpix/master

Fixed typo about boot signature and sectors 511 and 512