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>

Documentation/config.txt

@@ -559,8 +559,9 @@ core.whitespace::
 * `space-before-tab` treats a space character that appears immediately
   before a tab character in the initial indent part of the line as an
   error (enabled by default).
-* `indent-with-non-tab` treats a line that is indented with 8 or more
-  space characters as an error (not enabled by default).
+* `indent-with-non-tab` treats a line that is indented with space
+  characters instead of the equivalent tabs as an error (not enabled by
+  default).
 * `tab-in-indent` treats a tab character in the initial indent part of
   the line as an error (not enabled by default).
 * `blank-at-eof` treats blank lines added at the end of file as an error