build: Update HeAP placer option name to the final merged version

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

build/project-rules.mk

@@ -13,7 +13,7 @@ ICEPROG ?= iceprog
 IVERILOG ?= iverilog
 
 ifeq ($(PLACER),heap)
-NEXTPNR_SYS_ARGS += --heap-placer
+NEXTPNR_SYS_ARGS += --placer heap
 endif
 
 ICE40_LIBS ?= $(shell yosys-config --datdir/ice40/cells_sim.v)