Update README.md
Update README.md
`Git.init "a.git", bare:true` should create "a.git" bare repository

`Git.init "a.git", bare:true` should create "a.git" bare repository
instead of "a.git/.git" bare repository,
because `$ git init a.git --bare` create "a.git" bare repository.
Otherwise, we have to call `Git.init "a.git", bare:true, repository:"a.git"`.