Update experiment details of MuJoCo benchmark (#779)

Update the downloading url of the training logs and saved checkpoints
for MuJoCo tasks.

examples/mujoco/README.md

@@ -321,7 +321,7 @@ For pretrained agents, detailed graphs (single agent, single game) and log detai
 
 <a name="footnote1">[1]</a>  Supported environments include HalfCheetah-v3, Hopper-v3, Swimmer-v3, Walker2d-v3, Ant-v3, Humanoid-v3, Reacher-v2, InvertedPendulum-v2 and InvertedDoublePendulum-v2. Pusher, Thrower, Striker and HumanoidStandup are not supported because they are not commonly seen in literatures.
 
-<a name="footnote2">[2]</a>  Pretrained agents, detailed graphs (single agent, single game) and log details can all be found at [https://cloud.tsinghua.edu.cn/d/f45fcfc5016043bc8fbc/](https://cloud.tsinghua.edu.cn/d/f45fcfc5016043bc8fbc/).
+<a name="footnote2">[2]</a>  Pretrained agents, detailed graphs (single agent, single game) and log details can all be found at [Google Drive](https://drive.google.com/drive/folders/1IycImzTmWcyEeD38viea5JHoboC4zmNP?usp=share_link).
 
 <a name="footnote3">[3]</a>  We used the latest version of all mujoco environments in gym (0.17.3 with mujoco==2.0.2.13), but it's not often the case with other benchmarks. Please check for details yourself in the original paper. (Different version's outcomes are usually similar, though)