diff: 'diff.indentHeuristic' is no longer experimental

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>