Improve example

It was not immediately clear that the evaluation of `assert` statements can be
toggled from the `dart` binary.

Author: haarts

example/lib/main.dart

@@ -9,6 +9,8 @@ var loggerNoStack = Logger(
 );
 
 void main() {
+  print(
+      "Run with either `dart example/lib/main.dart` or `dart --enable-asserts example/lib/main.dart`.");
   demo();
 }
 
@@ -22,6 +24,4 @@ void demo() {
   logger.e("Error! Something bad happened", "Test Error");
 
   loggerNoStack.v({"key": 5, "value": "something"});
-
-  print("Test\nTest2");
 }