Docs: Installation for MacOS + conda says to use Python 3.8, but installation fails unless a newer version of python is used

### Describe the bug
Follow the anaconda instructions from the installation page, which says to use this command:

`conda create -n manim python=3.8`

This will fail, but this will succeed:

`conda create -n manim`

The installation fails when running:
`pip install -e .`

after the fix proposed above, this command must be:
`pip3 install -e .`