fix terminology: "PEP 8" the doc; "pep8" the script

PEPs aren't normally referred to with a dash (e.g. "PEP-8"), but rather with a space.
    (Note how PEPs cross-reference each other.)
"PEP-08" is completely nonstandard.
    Python 2 PEPs run into the triple-digits; Python 3 PEPs are/were 4-digit (3xxx).
    So padding it to 2 digits is completely weird.
Merge pull request #4401 from cvrebert/patch-1

fix terminology: "PEP 8" the document; "pep8" the script