Break the command blocks on README

[guavapaste1]

Pull request title

• I read contributing guideline
• I didn't find a similar pull request already open.
• My PR is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

Description

While installing Spleeter for the first time, I had to come back to the README multiple times just to copy the commands. GitHub has a nice button on code blocks that allow me to copy it, but since they are all in one block, I still had to select the command that I wanted to copy.

This pretty small change would make it easier to copy individual commands on the README.

How this patch was tested

You tested it, right?

I haven't touched any code, so I don't think testing is necessary.

• I implemented unit test whicn ran successfully using poetry run pytest tests/
• Code has been formatted using poetry run black spleeter
• Imports has been formatted using `poetry run isort spleeter``

Documentation link and external references

Please provide any info that may help us better understand your code.