Add missing <placeholder> for -display

Before this change, the documentation for the command-line options that
take arguments was inconsistent. There was a <placeholder> in the
“option” cell for almost all of the command-line options that take
arguments. -display was the only command-line option that takes
arguments that didn’t have a <placeholder>. This change makes the
documentation more consistent by ensuring that all command-line options
that take arguments have <placeholders>.

USAGE.md

@@ -87,7 +87,7 @@ Here brief usage of command line options, supported by game.
 | Option                                         | Type    | Default                             | Platform | Description                                                       |
 |------------------------------------------------|---------|-------------------------------------|----------|-------------------------------------------------------------------|
 | `-dedicated <config file>`, `-d <config file>` | path    | None                                | all      | Run game in dedicated mode                                        |
-| `-display`                                     | integer | 0                                   | all      | Run game on the selected display                                  |
+| `-display <number>`                            | integer | 0                                   | all      | Run game on the selected display                                  |
 | `-fullscreen`, `-f`                            | boolean | On                                  | all      | Run game in fullscreen mode                                       |
 | `-logfile`                                     | boolean | Off                                 | all      | Enable file logging to Descent3.log                               |
 | `-loglevel <LEVEL>`                            | string  | INFO (on Release), DEBUG (on Debug) | all      | Set log level (NONE, VERBOSE, DEBUG, INFO, WARNING, ERROR, FATAL) |