readme: fix typo ICEBOX -> ICESTORM
readme: fix typo ICEBOX -> ICESTORM

SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr