Documentation: indent-with-non-tab uses "equivalent tabs" not 8

Documentation: indent-with-non-tab uses "equivalent tabs" not 8

Update the documentation of the core.whitespace option
"indent-with-non-tab" to correctly reflect that it catches the use of
spaces instead of the equivalent tabs, rather than a fixed number.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>