Update dev-install.md

[UrszulaNeuman]

Use 'python' instead of 'python3'.
'python3' is python3.6
'python' is python3.11

[GDYendell]

Re-ran as the first build failed with

502 Server Error: Bad Gateway for url: [https://github.com/orgs/DiamondLightSource/packages/container/package/python-copier-template-example](https://github.com/orgs/DiamondLightSource/packages/container/package/python-copier-template-example/nbuild)

for some reason.

[coretl]

On ubuntu and derivatives there is no python, only python3, so I'd rather not change this. Could this instead be a note that if there are multiple pythons on your system then you may need to use python3.11 rather than python3 to get the correct version?

[AlexanderWells-diamond]

I agree with @coretl - it isn't even consistent at Diamond let alone across OSs. For example my machine has "python" as version 3.8.17.

[coretl]

Having examined a few more systems I'm wondering if this might be the right change after all. We change python3 to python in all places throughout the repo, but put a note before each bit of docs where we create a venv saying that on some machines you may need to use python3 or python3.11 instead to get the desired version.

@UrszulaNeuman do you have time to make this change to all the markdown files in this repo?