Fix typo in tutorial

I fixed typo and wrong representation of tree mode in tutorial.
`htc` is a obvious typo of `hct` and `hct.mode` is an octal value, so it must be 16384, not 040000 without `oct()`.

( Thanks to @Kroisse - https://github.com/gitpython-developers/GitPython/pull/115/files#pullrequestreviewcomment-5469388 )