doc: do not use `rm .git/index` when normalizing line endings

When illustrating how to normalize the line endings, the documentation in gitattributes tells the user to `rm .git/index`. This is incorrect for two reasons: - Users shouldn't be instructed to mess around with the internal implementation of Git using raw file system tools like `rm`. - Within a submodule or an additional working tree `.git` is just a file containing a `gitdir: <path>` pointer into the real `.git` directory. Therefore `rm .git/index` does not work. The purpose of the `rm .git/index` instruction is to remove all entries from the index without touching the working tree. The way to do this with Git is to use `read-tree --empty`. Signed-off-by: Andreas Heiduk <asheiduk@gmail.com> Helped-by: Junio C Hamano <gitster@pobox.com> Helped-by: Torsten Bögershausen <tboegi@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>