fix: typo (#500)

classNames 中的 root 的值应该是字符串，而不是对象

Author: WLyKan

src/Tooltip.tsx

@@ -32,7 +32,7 @@ export interface TooltipProps
   overlay: (() => React.ReactNode) | React.ReactNode;
   /** @deprecated Please use `styles={{ root: {} }}` */
   overlayStyle?: React.CSSProperties;
-  /** @deprecated Please use `classNames={{ root: {} }}` */
+  /** @deprecated Please use `classNames={{ root: '' }}` */
   overlayClassName?: string;
   getTooltipContainer?: (node: HTMLElement) => HTMLElement;
   destroyOnHidden?: boolean;