Documentation/git-rev-parse.txt: fix example in SPECIFYING RANGES.

Documentation/git-rev-parse.txt: fix example in SPECIFYING RANGES.

Please see http://bugs.debian.org/404795:

 In git-rev-parse(1), there is an example commit tree, which is used twice.
 The explanation for this tree is very clear: B and C are commit *parents* to
 A.

 However, when the tree is reused as an example in the SPECIFYING RANGES, the
 manpage author screws up and uses A as a commit *parent* to B and C!  I.e.,
 he inverts the tree.

 And the fact that for this example you need to read the tree backwards is
 not explained anywhere (and it would be confusing even if it was).

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>