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>

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