docs(only_os): name the field it really matches, and drop the RE2 lookahead

[ci-robbot]

only_os is matched against sysinfo's OS.Name, and sysinfo puts os-release's
PRETTY_NAME there. ID goes to OS.Vendor, which nothing reads
(zcalusic/sysinfo@v1.1.3/os.go). The README says the opposite, so:

• every lowercase example in it (only_os: "ubuntu|opensuse-leap",
  only_os: "ubuntu") matches nothing. A reader who copies one gets a stage
  that never runs, and the debug line says "doesn't match os name", naming a
  string they did not write a pattern for;
• only_os: "^(?!ubuntu).*" is worse than not matching. Go's regexp is RE2,
  which has no lookahead, so it fails to compile and OnlyIfOS returns the
  compile error instead of an OS verdict.

Checked on this host (ID=alpine, PRETTY_NAME="Alpine Linux v3.24"):

"ubuntu|opensuse-leap"   vs "Ubuntu 24.04.1 LTS"   -> false
"^(?!ubuntu).*"          compile error: invalid or unsupported Perl syntax: `(?!`
"Ubuntu.*"               vs "Ubuntu 24.04.1 LTS"   -> true

only_os_version was documented correctly: VERSION_ID does land in
OS.Version.

Tests. pkg/plugins/if_os_fields_test.go:

• two specs that read the host's own /etc/os-release and assert only_os
  matches its PRETTY_NAME and does not match its ID, skipping if the host
  cannot tell the two apart. Pointing OnlyIfOS at OS.Vendor fails both;
• one that only_os_version matches VERSION_ID;
• one per filter key that compiles every pattern the README documents, so a
  documented lookahead cannot stay documented. Restoring ^(?!ubuntu).* fails
  it.

Found while migrating kairos-io/kairos's bundled cloudconfigs onto only_os
(kairos-io/kairos#4696), where the docs sent me to the wrong field.