Update README.md

The first letter of "pod" should be lowercase.

```
pod update

[!] Invalid `Podfile` file: undefined method `Pod' for Podfile:Pod::Podfile. Updating CocoaPods might fix the issue.

 #  from /xxxxx/myDemo/Podfile:20
 #  -------------------------------------------
 #  pod 'pop'
 >  Pod 'VBFPopFlatButton'
 #  
 #  -------------------------------------------
```