doc: change type of child_process.signalCode to string

During testing, and interfacing with child_processes, I found that the
child_process.signalCode property to be the string representation of
the signal, not number.

PR-URL: https://github.com/nodejs/node/pull/35223


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>