cmd/ethkey: fix the README to match updated commands (#17332)

cmd/ethkey/README.md

@@ -21,21 +21,33 @@ Private key information can be printed by using the `--private` flag;
 make sure to use this feature with great caution!
 
 
-### `ethkey sign <keyfile> <message/file>`
+### `ethkey signmessage <keyfile> <message/file>`
 
 Sign the message with a keyfile.
 It is possible to refer to a file containing the message.
+To sign a message contained in a file, use the `--msgfile` flag.
 
 
-### `ethkey verify <address> <signature> <message/file>`
+### `ethkey verifymessage <address> <signature> <message/file>`
 
 Verify the signature of the message.
 It is possible to refer to a file containing the message.
+To sign a message contained in a file, use the --msgfile flag.
+
+
+### `ethkey changepassphrase <keyfile>`
+
+Change the passphrase of a keyfile.
+use the `--newpasswordfile` to point to the new password file.
 
 
 ## Passphrases
 
 For every command that uses a keyfile, you will be prompted to provide the 
 passphrase for decrypting the keyfile.  To avoid this message, it is possible
-to pass the passphrase by using the `--passphrase` flag pointing to a file that
+to pass the passphrase by using the `--passwordfile` flag pointing to a file that
 contains the passphrase.
+
+## JSON
+
+In case you need to output the result in a JSON format, you shall by using the `--json` flag.