Typo in README

[prakharb10]

I came across this when I was working on another issue. I think there's a minor typo in the instructions to run the tests:

openqaoa/README.md

Line 101 in 2cc9ea0

To run the unit-tests, first, make sure to have installed all the optional testing dependencies by running `make dev-install-tests`. Next type `pytest tests/ /src/*/tests/` from the project's root folder. This runs the common metapackage unit-tests and the unit-tests for each OpenQAOA plugin.

It should be: pytest tests/ src/*/tests/