Update external_completers.md

[SLASHLogin]

Warning: nu::parser::deprecated

  ⚠ Flag deprecated.
    ╭─[/home/slashlogin/.config/nushell/config.nu:34:11]
 33 │     | where name == $spans.0
 34 │     | get -i 0.expansion
    ·           ─┬
    ·            ╰── get --ignore-errors was deprecated in 0.106.0 and will be removed in a future release.
 35 │
    ╰────
  help: This flag has been renamed to `--optional (-o)` to better reflect its behavior.

[ayax79]

Thanks!