Fix units in sizeof_fmt

[cbrnr]

Fixes #12961.

[agramfort]

to me MB and GB is much more standard. eg https://www.vaia.com/en-us/explanations/computer-science/computer-organisation-and-architecture/units-of-data-storage/#:~:text=The%20different%20units%20of%20data%20storage%20in%20computer%20science%20include,%2C%20and%20Yottabyte%20(YB).

[cbrnr]

Yes, but then we have to switch to base 1000, because currently we are using base 1024 (e.g. 1 kB = 1000 bytes, but 1 kiB = 1024 bytes). I'm fine either way.

[drammock]

This was discussed before in regards to the sys_info output and we settled on GiB so let's be consistent and stick with that here.

[drammock]

LGTM, just one capitalization fix in the changelog entry

[cbrnr]

I'll fix the tests this afternoon!

[cbrnr]

I will definitely never understand Sphinx, so I'd be happy if someone fixed this reference not found issue for me. Thanks!