mock-alu: update README.md after asap7 reorganization

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

flow/designs/src/mock-alu/src/test/resources/README.md

@@ -2,5 +2,5 @@ ASAP7 behavioral model files
 ============================
 
 To run unit-tests, copy the behavioral model Verilog files from ASAP7
-from https://github.com/The-OpenROAD-Project/asap7/tree/master/asap7sc7p5t_28/Verilog
+from https://github.com/The-OpenROAD-Project/asap7sc7p5t_28/tree/main/Verilog
 into this folder.