chore(state): remove unnecessary character

- Remove unnecessary `'` character

Closes #2113

Author: djindjic

src/state.js

@@ -172,7 +172,7 @@ function $StateProvider(   $urlRouterProvider,   $urlMatcherFactory) {
 
     var name = state.name;
     if (!isString(name) || name.indexOf('@') >= 0) throw new Error("State must have a valid name");
-    if (states.hasOwnProperty(name)) throw new Error("State '" + name + "'' is already defined");
+    if (states.hasOwnProperty(name)) throw new Error("State '" + name + "' is already defined");
 
     // Get parent name
     var parentName = (name.indexOf('.') !== -1) ? name.substring(0, name.lastIndexOf('.'))