Fix typo in lib.dom.d.ts

[mlhaufe]

Ref: https://github.com/Microsoft/TypeScript/blob/master/lib/lib.dom.d.ts#L14582

interface DocumentOrShadowRoot {
    ...
    readonly stylesheets: StyleSheetList;
    ...
}

The property stylesheets should be camelCase styleSheets as per: http://w3c.github.io/webcomponents/spec/shadow/#extensions-to-the-documentorshadowroot-mixin