[Doc] Typo in README.md

[SchrodingersGat]

There is a simple typo in the README file:

Extensions
As a last step, we need to build the extensions.

cd into extensions

qmake src.pro

qmake CONFIG+=Release extensions.pro

Here qmake src.pro should read qmake extensions.pro

[mrbean-bremen]

Thanks! Do you want to make a PR?

[SchrodingersGat]

I was hoping to avoid it ;)

#257

[mrbean-bremen]

It wasn't mandatory - just wanted to give you the opportunity :)

[SchrodingersGat]

No worries. Thanks for the awesome project!