Fix erroneous missing required option error message

Probably a copy/paste error since the messages for the default command say "Missing required option" vs "Missing required argument"

Author: 0xced

src/Spectre.Console.Cli.Tests/CommandAppTests.Options.cs

@@ -21,7 +21,7 @@ public void Should_Throw_If_Required_Option_Is_Missing()
             // Then
             result.ShouldBeOfType<CommandRuntimeException>()
                 .And(ex =>
-                    ex.Message.ShouldBe("Command 'test' is missing required argument 'foo'."));
+                    ex.Message.ShouldBe("Command 'test' is missing required option 'foo'."));
         }
     }
 }

src/Spectre.Console.Cli/CommandRuntimeException.cs

@@ -44,7 +44,7 @@ internal static CommandRuntimeException MissingRequiredOption(CommandTree node,
             return new CommandRuntimeException($"Missing required option '{option.GetOptionName()}'.");
         }
 
-        return new CommandRuntimeException($"Command '{node.Command.Name}' is missing required argument '{option.GetOptionName()}'.");
+        return new CommandRuntimeException($"Command '{node.Command.Name}' is missing required option '{option.GetOptionName()}'.");
     }
 
     internal static CommandRuntimeException NoConverterFound(CommandParameter parameter)