docs: fix CocoaPods img shield

Author: muyexi

README.md

@@ -1,5 +1,5 @@
 
-[![CocoaPods](https://img.shields.io/cocoapods/v/StaticTableViewController.svg?maxAge=2592000)](http://cocoadocs.org/docsets/StaticTableViewController)
+[![CocoaPods](https://img.shields.io/cocoapods/v/StaticTableViewController.svg)](https://img.shields.io/cocoapods/v/StaticTableViewController.svg)
 [![Carthage Compatible](https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat)](https://github.com/Carthage/Carthage)
 [![license](https://img.shields.io/github/license/mashape/apistatus.svg?maxAge=2592000)](https://github.com/muyexi/StaticTableViewController/blob/master/LICENSE)