Typo in the basic Readme

[bentxt]

Nothing bad just a little confusing for a newcomer

in https://github.com/rescript-lang/create-rescript-app/blob/master/templates/rescript-template-basic/README.md

Does Rescript generate a Commonjs module here? Because the file extension is 'mjs'

So you have to change this lines from:

## Run
 
` ` `sh
 node src/Demo.res.js
` ` `

into:

 ## Run
 
` ` `sh
 node src/Demo.res.mjs
` ` `

(I've put whitespace between backticks, please remove them)