Fixing custom serializers example (#78)

The current example invokes the ExpressPinoLogger constructor twice, which throws in init:

```
TypeError: Cannot set property 'log' of undefined
    at loggingMiddleware (/repos/app/node_modules/pino-http/logger.js:99:23)
```

Fix for #76

README.md

@@ -57,7 +57,7 @@ npm i express-pino-logger --save
 'use strict'
 
 var app = require('express')()
-var pino = require('express-pino-logger')()
+var pino = require('express-pino-logger')
 
 app.use(pino)