Remove incorrect information about HEAD

The statement that the working directory (aside: "working tree" would be
a more git-like term) will always match the current state of HEAD is
completely incorrect.  That would imply that changes to files get in
some way committed as files are saved.

Even the second part of the paragraph is incorrect.  Changing HEAD does
not necessarily change the contents of the working tree as in `git
reset` without the `--hard` option.