README.md edit | Fix import in readme.md

[alenthomas]

fixes import statement in the usage section of readme

[nikitaeverywhere]

Please state two imports like this:

import { XMasonry, XBlock } from "react-xmasonry";             // Imports JSX plain sources
import { XMasonry, XBlock } from "react-xmasonry/dist/index";  // Imports precompiled bundle

[alenthomas]

I've modified and added both imports