whitespace: fix config.txt description of indent-with-non-tab

Fix garbled description.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -307,7 +307,7 @@ core.whitespace::
   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 that can be replaced with tab characters.
+  space characters as an error (not enabled by default).
 
 alias.*::
 	Command aliases for the gitlink:git[1] command wrapper - e.g.