doc: fix external links with 404 status
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
Fixes: https://github.com/nodejs/node/issues/15462
uname(3)
links seem to be autogenerated (to 404 http://man7.org/linux/man-pages/man3/uname.3.html), so maybe we should check something in doc scripts.