Update README.md

Author: arthur3486

README.md

@@ -8,7 +8,7 @@
 [ ![Download](https://api.bintray.com/packages/arthurimsacc/maven/mvvm-core/images/download.svg) ](https://bintray.com/arthurimsacc/maven/mvvm-core/_latestVersion)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![Platform](https://img.shields.io/badge/platform-Android-green.svg)](http://developer.android.com/index.html)
-![](https://img.shields.io/badge/API-21%2B-green.svg?style=flat)
+![](https://img.shields.io/badge/API-18%2B-green.svg?style=flat)
 ![](https://travis-ci.org/arthur3486/android-mvvm.svg?branch=master)
 
 ## Contents