Fix TypeScript declaration.

Fixes error:

src/index.d.ts(6,60): error TS2371: A parameter initializer is only allowed in a function or constructor implementation.

Author: bradleyayers

src/index.d.ts

@@ -3,4 +3,4 @@ import * as React from 'react';
 type CustomElementCtor = { new(...args: any[]): HTMLElement };
 type ReactComponentCtor = { new(...args: any[]): React.Component<any, any> };
 type Options = { React?: any, ReactDOM?: any };
-export default function (CustomElement: CustomElementCtor, opts: Options = {}): ReactComponentCtor;
+export default function (CustomElement: CustomElementCtor, opts: Options = {}): ReactComponentCtor