Use a https schema so the example works

When you try and run the example with the latest version of the lib it throws the following. This PR changes the example to use https, which works.

```
assert.js:93
  throw new assert.AssertionError({
        ^
AssertionError: must have a URL that starts with http: or https:
```

Author: Adam Baldwin

README.md

@@ -9,7 +9,7 @@ It handles all the caching and HTTP calls.
 ```javascript
 var RegClient = require('npm-registry-client')
 var client = new RegClient(config)
-var uri = "npm://registry.npmjs.org/npm"
+var uri = "https://registry.npmjs.org/npm"
 var params = {timeout: 1000}
 
 client.get(uri, params, function (error, data, raw, res) {