chore(docs): Fixed example typo & code style in npm-scripts.md

[NNNaix]

Fixed example typo & code style in npm-scripts.md

First of all, thanks for your works.
When I read npm docs about using npm/scripts, I find a typo in the "Examples" section.
the scripts code example is as follows:

{ 
    "scripts" : { 
        "install" : "scripts/install.js", 
        "postinstall" : "scripts/postinstall.js", 
        "uninstall" : "scripts/uninstall.js"
    }
}

Considering the context：

then scripts/install.js will be called for the install and post-install stages of the lifecycle, and scripts/uninstall.js will be called when the package is uninstalled. Since scripts/install.js is running for two different phases, it would be wise in this case to look at the npm_lifecycle_event environment variable.

I think the postinstall script command should be "scripts/install.js", and the official website documentation also verified my idea.

By the way also beautify the code style of the JSON example :P