Remove invalid blog links

Author: maryamziaa

com.unity.ml-agents/Documentation~/Blog-posts.md

@@ -4,19 +4,15 @@ We have published a series of blog posts that are relevant for ML-Agents:
   [ML-Agents plays Dodgeball](https://blog.unity.com/technology/ml-agents-plays-dodgeball)
 - (May 5, 2021)
   [ML-Agents v2.0 release: Now supports training complex cooperative behaviors](https://blogs.unity3d.com/2021/05/05/ml-agents-v2-0-release-now-supports-training-complex-cooperative-behaviors/)
-- (November 20, 2020)(external link is ok???)
+- (November 20, 2020)
   [How Eidos-Montréal created Grid Sensors to improve observations for training agents](https://www.eidosmontreal.com/news/the-grid-sensor-for-automated-game-testing/)
 - (February 28, 2020)
   [Training intelligent adversaries using self-play with ML-Agents](https://blogs.unity3d.com/2020/02/28/training-intelligent-adversaries-using-self-play-with-ml-agents/)
 - (November 11, 2019)
   [Training your agents 7 times faster with ML-Agents](https://blogs.unity3d.com/2019/11/11/training-your-agents-7-times-faster-with-ml-agents/)
-- (October 2, 2018)(????)
-  [Puppo, The Corgi: Cuteness Overload with the Unity ML-Agents Toolkit](https://blogs.unity3d.com/2018/10/02/puppo-the-corgi-cuteness-overload-with-the-unity-ml-agents-toolkit/)
 - (June 26, 2018)
   [Solving sparse-reward tasks with Curiosity](https://blogs.unity3d.com/2018/06/26/solving-sparse-reward-tasks-with-curiosity/)
 - (June 19, 2018)
   [Unity ML-Agents Toolkit v0.4 and Udacity Deep Reinforcement Learning Nanodegree](https://github.com/udacity/deep-reinforcement-learning)
-- (December 11, 2017)(???)
-  [Using Machine Learning Agents in a real game: a beginner’s guide](https://blogs.unity3d.com/2017/12/11/using-machine-learning-agents-in-a-real-game-a-beginners-guide/)
 - (September 19, 2017)
   [Introducing: Unity Machine Learning Agents Toolkit](https://blogs.unity3d.com/2017/09/19/introducing-unity-machine-learning-agents/)