Install Redis before testing Celery

[qin-guan]

In the installation guide, Celery is tested to see if it able to connect to Redis, but will result in an error as the Redis package is not installed.

https://github.com/DMOJ/docs/blob/master/docs/site/installation.md?plain=1#L159

pip3 install Redis should be ran before the command.

I'd be happy to open a PR to fix this, thank you!

[int-y1]

pip install redis also works

i'm not sure which one is better