Cleanup uses of prihdr and header in the FITS docs

@MSeifert04 fixed an instance of variable names changing half way through the io.fits docs in one place - however there are other places where this happens, e.g. in the headers.rst file:

https://github.com/astropy/astropy/blame/master/docs/io/fits/usage/headers.rst#L138

Fundamentally the issue is that we are skipping the doctests in all these files, which is bad! We really should fix the examples so they actually run (and with data we can put on data.astropy.org, no excuse really)

I also think header makes more sense than prihdr as a variable name. It would be great to fix this soon since there have been some complaints/confusion.

[MSeifert04]

@astrofrog You’re right. I actually opened https://github.com/astropy/pytest-remotedata/issues/5 because that would allow to conditionally run the doctests when remote data is given.

I would actually hate to riddle the code with doctest: +REMOTE_DATA comments.

[MSeifert04]

thanks for the fast reply @drdavella , I’ll have a look at your PR and see how far I can get 😄