Change commands arg name to the singular form (#520)

As the plural "commands" suggest that it can take many commands while it only works with one.

Co-authored-by: Mike Donnalley <mdonnalley@salesforce.com>

src/commands/help.ts

@@ -2,7 +2,7 @@ import {Args, Command, Flags, loadHelpClass} from '@oclif/core'
 
 export default class HelpCommand extends Command {
   static args = {
-    commands: Args.string({description: 'Command to show help for.', required: false}),
+    command: Args.string({description: 'Command to show help for.', required: false}),
   }
 
   static description = 'Display help for <%= config.bin %>.'