Some typo and bug fix

[peacedog123]

Fix inconsistency of flags in multigroup example
Fix NodeHost.Stop() called twice problem

1. In the README.md
   The command has changed to ./example-multigroup -replicaid 1, so here change the code to make it consistent between code and README.md file.

2. When we input exit in the console, we will hit NodeHost.Stop() called twice panic like below:

exit
panic: NodeHost.Stop called twice

goroutine 1 [running]:
github.com/lni/dragonboat/v4.(*NodeHost).Close(0xc0001bea80)
        /<home-dir>/go/pkg/mod/github.com/lni/dragonboat/[email protected]/nodehost.go:390 +0x9d0
main.main()
        /<git-repo>/dragonboat-example/multigroup/main.go:195 +0x88c