support/scripts: fix graph-build-time help text

The graph-build-time help text currently looks like this:

usage: graph-build-time [-h] [--type GRAPH_TYPE] [--order GRAPH_ORDER]
                        [--alternate-colors] [--input OUTPUT] --output OUTPUT

Obviously, naming the parameter for --input as OUTPUT is not a very
good idea, so this commit fixes that to name it "INPUT", as expected.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>