nmap: add -O example (#4400)

pages/common/nmap.md

@@ -4,6 +4,10 @@
 > Some features only activate when Nmap is run with privileges.
 > More information: <https://nmap.org>.
 
+- Check if an IP address is up, and guess the remote host's operating system:
+
+`nmap -O {{ip_or_hostname}}`
+
 - Try to determine whether the specified hosts are up and what are their names:
 
 `nmap -sn {{ip_or_hostname}} {{optional_another_address}}`