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

PEPs aren't normally referred to with a dash (e.g. "PEP-8"), but rather with a space. (Look at how actual 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.

Uses `pep8` invariantly for the command.