Documentation typo in prerequisites section

[Piyush Nalawade]

Describe the bug
In the prerequisites section of readme.md the formula for macOS is brew install postgress which has a typo for postgres.

To Reproduce
Steps to reproduce the behaviour:

1. Go to 'README.md'
2. Scroll down to 'Prerequisites -> MacOS'
3. See type

Expected behaviour
Typo should be fixed to postgres.

[Piyush Nalawade]

I will put up a pull request for this.