diff: 'diff.indentHeuristic' is no longer experimental

The indent heuristic started out as experimental, but it's now our default diff heuristic since 33de716387 (diff: enable indent heuristic by default, 2017-05-08). Alas, that commit didn't update the documentation, and the description of the 'diff.indentHeuristic' configuration variable still implies that it's experimental and not the default. Update the description of 'diff.indentHeuristic' to make it clear that it's the default diff heuristic. The description of the related '--indent-heuristic' option has already been updated in bab76141da (diff: --indent-heuristic is no longer experimental, 2017-10-29). Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>