doc: DIE_AREA is shorthand for a method to specify floorplan

There is an error message if CORE_AREA is not specified together
with DIE_AREA, but it is close enough to the truth to say that
DIE_AREA is a method to specify floorplan.

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

docs/user/FlowVariables.md

@@ -275,7 +275,7 @@ configuration file.
 | `MACRO_HALO_X`         | Set macro halo for x-direction. Only available for ASAP7 PDK. |
 | `MACRO_HALO_Y`         | Set macro halo for y-direction. Only available for ASAP7 PDK. |
 
-The various methods to specify the die and core area(`FLOORPLAN_DEF`, `FOOTPRINT`, `DIE_AREA`, `CORE_AREA` and `CORE_UTILIZATION`) are mutually exclusive. If two methods are specified, floorplan.tcl will exit with an error requiring that a single method is used.
+The various methods to specify the die and core area(`FLOORPLAN_DEF`, `FOOTPRINT`, `DIE_AREA` and `CORE_UTILIZATION`) are mutually exclusive. If two methods are specified, floorplan.tcl will exit with an error requiring that a single method is used.
 
 #### Placement