Add missing article to example.

docs/client/api.html

@@ -1421,7 +1421,7 @@ Example:
         return "Welcome to Awesome Town, " + user.profile.name;
     };
     Accounts.emailTemplates.enrollAccount.text = function (user, url) {
-       return "You have been selected to participate in building better future!"
+       return "You have been selected to participate in building a better future!"
          + " To activate your account, simply click the link below:\n\n"
          + url;
     };