PEP 818: Fix typos

[davidbrochart]

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

[Rosuav]

Not sure why the build's failing but it shouldn't be caused by these fixes, which look fine to me.

[StanFromIreland]

They're failing because the docs are currently down.

[hugovk]

See python/cpython#158554.

[hoodmane]

Thanks @davidbrochart!