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: nodejs#35223
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: Linn Dahlgren

doc/api/child_process.md

@@ -1389,9 +1389,9 @@ connection to the child.
 
 ### `subprocess.signalCode`
 
-* {integer}
+* {string|null}
 
-The `subprocess.signalCode` property indicates the signal number received by
+The `subprocess.signalCode` property indicates the signal received by
 the child process if any, else `null`.
 
 ### `subprocess.spawnargs`