[copy-webpack-plugin] fix transform parameter
[copy-webpack-plugin] fix transform parameter

the `content` parameter of `transform` is a `Buffer`, not a `string`.
mcliment/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped