Update installing requirements section

[BakungaBronson]

Changed the command to install all the requirements and not just litellm.

[jamiekawabata]

Maybe also change the "Python 3.7 or higher" prerequisite.

I think litellm requires Python 3.8 at a minimum.

The packages in the requirements.txt do not all work on Python 3.8, so demanding everything from requirements.txt would further raise the python requirement.

The readme could strike the explicit python prerequisite and people are on their own.

Edit: It looks like Python 3.9 is enough to install everything in requirements.txt

[BakungaBronson]

I use Python 3.10, so I was oblivious to this. Thank you for pointing it out. I have added the edit too.