Corrected variable reference in the README install instructions.

sky130/README

diff --git a/sky130/README b/sky130/README
index 00b9851..8941de1 100644
--- a/sky130/README
+++ b/sky130/README
@@ -18,7 +18,7 @@
 
     This repository may go in any convenient location.  The Makefile suggests
     the target location ~/projects/foundry/skywater-pdk but any location will
-    do as long as the definition for CYPRESS_PATH in the Makefile is set
+    do as long as the definition for SKYWATER_PATH in the Makefile is set
     appropriately.
 
     So cd to the target location parent directory (e.g., "cd ~/projects/foundry")