update to readme file

making changes to the version typo  since newbies have a problem when pasting to pubspec.yaml file. The version should be ^3.0.6 instead of just 3.0.6

README.md

@@ -32,7 +32,7 @@ pub.dartlang.org: (you can use 'any' instead of a version if you just want the l
 
 ```yaml
 dependencies:
-  uuid: 3.0.6
+  uuid: ^3.0.6
 ```
 
 ```dart