fix(readme): changed file variable name to path
fix(readme): changed file variable name to path

Having the variable name as 'file' caused the mtd file to be always
named as 'undefined.mtd' since it was not able to find 'path' on
'src/initParams.js:7'. This may cause issues when multiple files are
being downloaded concurrently to same location.

tusharmath/Multi-threaded-downloader,continuata/Multi-threaded-downloader