Fix typos in readme

[cypherhoodlum]

readme.md has instructions to use the command repo permissions when it should be repo permission.

From main.go:

switch cmd {
	case "repo":
		subcmd := os.Args[2]
		switch subcmd {
		case "create":
			repoCreate(cfg, pool)
		case "clone":
			repoClone(cfg, pool)
		case "permission": // relevant case here
			repoPermission(cfg, pool)
		default:
			log.Fatalf("unknown repo sub command %v", subcmd)
		}