doc: update cross compiler machine for Linux armv7

Update cross compiler machine for Linux armv7 to Ubuntu 18.04.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/32812
Refs: https://github.com/nodejs/build/issues/2168


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>