"pmboostrap" typo in various parts of the code

I wanted to grep for "pmbootstrap" in the source code and was confused when I got so few results, then I realised I had made a typo 😅. Anyway, we should probably fix these typos in the code:

$ rg pmboostrap
docs/conf.py
58:htmlhelp_basename = "pmboostrapdoc"

docs/index.rst
4:pmboostrap is the central command-line application for postmarketOS development. Among other things,

docs/usage.rst
13:  $ pmboostrap init

docs/api/modules.rst
1:pmboostrap code documentation

pmb/helpers/exceptions.py
15:    could for example be raised if there is an error in a package pmboostrap is

README.md
51:Attempting to use pmboostrap with old postmarketOS versions (old pmaports