fix flutter readme pub banner

Author: lohanidamodar

templates/flutter/README.md.twig

@@ -1,6 +1,6 @@
 # {{ spec.title }} {{sdk.name}} SDK
 
-[![pub package](https://img.shields.io/pub/v/{{ language.params.packageName }}?style=flat-square.svg)](https://pub.dartlang.org/packages/{{ language.params.packageName }})
+[![pub package](https://img.shields.io/pub/v/{{ language.params.packageName }}?style=flat-square)](https://pub.dartlang.org/packages/{{ language.params.packageName }})
 ![License](https://img.shields.io/github/license/{{ sdk.gitUserName|url_encode }}/{{ sdk.gitRepoName|url_encode }}.svg?style=flat-square)
 ![Version](https://img.shields.io/badge/api%20version-{{ spec.version|url_encode }}-blue.svg?style=flat-square)
 {% if sdk.twitterHandle %}