doc: twister: fix argument order in custom flash example

Fixes sample command added to the documentation by commit
a31f784e ("twister: add a --flash-command flag")

During the review of #93944, I recommended to change the order and pass
user --flag(s) last. This was changed in the code but not in the example
added to the documentation.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>