fix: Update index.d.ts (#48)

Fixes typescript compile error: 

   File name '.../node_modules/@types/React/index.d.ts' differs from already included file name 
   '.../node_modules/@types/react/index.d.ts' only in casing.

Author: rizrmd

index.d.ts

@@ -1,7 +1,7 @@
 // Type definitions for react-virtualzed-tree
 // Definitions by: Diogo Cunha
 
-import * as React from 'React';
+import * as React from 'react';
 
 interface BasicNode {
   id: number | string;