Fix wrong comment syntax

Author: JasonEtco

README.md

@@ -8,7 +8,7 @@ $ npm install @probot/serverless-lambda
 ```
 
 ```javascript
-# handler.js
+// handler.js
 const { serverless } = require('@probot/serverless-lambda')
 const appFn = require('./')
 module.exports.probot = serverless(appFn)