system(3): Use plural form of 'command'

Pick the plural form in the sentence about running arbitrary commands in
the security consideration section.

PR:	    294497
Event:	    Halifax Hackathon 202606
Location:   Couch

lib/libc/stdlib/system.3

@@ -101,7 +101,7 @@ compatible.
 The
 .Fn system
 function is easily misused in a manner that enables a malicious
-user to run arbitrary command,
+user to run arbitrary commands,
 because all meta-characters supported by
 .Xr sh 1
 would be honored.