correction to example. see https://github.com/SymbiFlow/prjtrellis/pull/123#issuecomment-594201676

diff --git a/README.md b/README.md
index fc3be89..39bbb65 100644
--- a/README.md
+++ b/README.md

@@ -31,7 +31,7 @@
 Clone and install **latest git master** versions (Yosys 0.8 is not sufficient for ECP5 development) of [Yosys](https://github.com/YosysHQ/yosys)
  and [nextpnr](https://github.com/YosysHQ/nextpnr) according to their own instructions. Ensure
  to include the ECP5 architecture when building nextpnr; and point it towards your prjtrellis
- folder.  (for example: `cmake -DARCH=ecp5 -DTRELLIS_ROOT=/usr/share/trellis/ .`)
+ folder.  (for example: `cmake -DARCH=ecp5 -DTRELLIS_INSTALL_PREFIX=/usr/share/trellis/ .`)
 
 You should now be able to build the [examples](examples).