Update README.md for rapidus

From what I tested the export is missing a final step in the directory path.

Signed-off-by: Augusto Berndt <[email protected]>

Author: gudeh

flow/designs/rapidus2hp/README.md

@@ -7,7 +7,7 @@ The ORFS-specific files for the Rapidus platform are stored separately in the pr
 ```
 cd rapidus_platform_dir_goes_here
 git clone [email protected]:The-OpenROAD-Project-private/rapidus
-export PLATFORM_HOME=`pwd`/rapidus_platform_dir_goes_here
+export PLATFORM_HOME=`pwd`/rapidus_platform_dir_goes_here/rapidus
 ```
 
 For more information, check out (http://github.com/The-OpenROAD-Project-private/rapidus)