Sync program help documentation with README

For both Windows and macOS, Visual Studio is launched by default. The readme correctly states this but the program help wasn't updated in microsoft#492

src/Microsoft.VisualStudio.SlnGen/ProgramArguments.cs

@@ -178,7 +178,7 @@ public ProgramArguments()
             "--launch",
             CommandOptionType.MultipleValue,
             ValueName = "true|false",
-            Description = "Launch Visual Studio after generating the Solution file.  Default: true on Windows")]
+            Description = "Launch Visual Studio after generating the Solution file.  Default: true on Windows and macOS")]
         public string[] LaunchVisualStudio { get; set; }
 
         /// <summary>
@@ -548,4 +548,4 @@ private bool GetBoolean(string[] values, bool defaultValue = false)
             return null;
         }
     }
-}
+}