minor grammatical + spelling changes in README

[realytcracker]

minor grammatical + spelling changes in README

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

Description

minor changes made to the README, fixing grammatical and spelling errors.

How this patch was tested

You tested it, right?

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

Documentation link and external references

no code changes, just grammar and spelling.