fix user agent

Author: lohanidamodar

templates/dart/lib/src/client_io.dart.twig

@@ -43,7 +43,7 @@ class ClientIO extends ClientBase with ClientMixin {
       'x-sdk-platform': '{{ sdk.platform }}',
       'x-sdk-language': '{{ language.name | caseLower }}',
       'x-sdk-version': '{{ sdk.version }}',
-      'user-agent' : '{{spec.title | caseUcfirst}}{{ language.name | caseUcfirst }}SDK/{{ sdk.version }} ({{Platform.operatingSystem}}; {{Platform.operatingSystemVersion}})',
+      'user-agent' : '{{spec.title | caseUcfirst}}{{ language.name | caseUcfirst }}SDK/{{ sdk.version }} (${Platform.operatingSystem}; ${Platform.operatingSystemVersion})',
 {% for key,header in spec.global.defaultHeaders %}
       '{{key}}' : '{{header}}',
 {% endfor %}