Update README TRELLIS_INSTALL_PREFIX example 
Update README TRELLIS_INSTALL_PREFIX example 

per https://github.com/YosysHQ/nextpnr/pull/406#discussion_r387937313 it is not an install directory
YosysHQ/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,SymbiFlow/nextpnr,SymbiFlow/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr