[libs][librrgraph] fix README

Author: amin1377

libs/librrgraph/src/io/gen/README.gen.md

@@ -2,4 +2,4 @@
 `rr_graph_uxsdcxx_interface.h` are generated via uxsdcxx and are checked in to
 avoid requiring python3 and the uxsdcxx depedencies to build VPR.
 
-See `vpr/src/route/gen/SCHEMA_GENERATOR.md` for details.
+See `$VTR_DIR/libs/librrgraph/src/base/SCHEMA_GENERATOR.md` for details.