update FlowVariables.md

Signed-off-by: Eder Monteiro <[email protected]>

Author: eder-matheus

docs/user/FlowVariables.md

@@ -357,7 +357,6 @@ configuration file.
 - [FLOORPLAN_DEF](#FLOORPLAN_DEF)
 - [GPL_ROUTABILITY_DRIVEN](#GPL_ROUTABILITY_DRIVEN)
 - [GPL_TIMING_DRIVEN](#GPL_TIMING_DRIVEN)
-- [HOLD_SLACK_MARGIN](#HOLD_SLACK_MARGIN)
 - [IO_PLACER_H](#IO_PLACER_H)
 - [IO_PLACER_V](#IO_PLACER_V)
 - [MATCH_CELL_FOOTPRINT](#MATCH_CELL_FOOTPRINT)