show help for invalid subcommand

[itsspriyansh]

Display help if an invalid subcommand is passed.

[garg3133]

Two things:

• It's possible that the user didn't want to pass a subcommand and only passed it by mistake. To take that case into account as well, add the following line at the end: "For complete Android help, run: npx @nightwatch/mobile-helper android --help".
• Replace [subcmd] in usage with just subcmd. [] blocks are used to represent optional arguments, but in the case of subcommands, passing the subcommand to use would not be optional.

[itsspriyansh]

@garg3133 Changes done and updated the preview image.

[itsspriyansh]

@garg3133 Can we get this merged now?