minor grammar and spelling fixes in CONTRIBUTING.md

[realytcracker]

minor grammar and spelling fixes in CONTRIBUTING.md

• [ 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 grammar and spelling fixes on the contribution guidelines.

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 were made.