react-lazyload: Improve typing of placeholder
react-lazyload: Improve typing of placeholder

georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped