avoid raw github URLs for ontologies in the spec

[cmungall]

Under https://hupo-psi.github.io/mzTab/2_0-metabolomics-release/mzTab_format_specification_2_0-M_release.html#the-psi-mass-spectrometry-controlled-vocabulary-cv

You have rows like this:

Cell Type ontology (https://raw.githubusercontent.com/obophenotype/cell-ontology/master/cl-basic.obo).

For OBO ontologies, always use an OBO PURL for the content. These are guaranteed stable, whereas the above URL will break if we reorganize our repo structure or move away from github

For a web page describing an ontology, every OBO has a URL of form https://obofoundry.org/ontology/cl

[nilshoffmann]

@cmungall Thank you for the pointer, we will update the spec doc for the next minor version release.