Update test_thread_print_fstrings.py

`clk + led` -> `cnt + led`

Author: shtaxxx

tests/extension/thread_/print_fstrings/test_thread_print_fstrings.py

@@ -116,7 +116,7 @@
           th_blink <= th_blink_9;
         end
         th_blink_9: begin
-          $display("clk + led = %d", (CLK + LED));
+          $display("cnt + led = %d", (CNT + LED));
           th_blink <= th_blink_10;
         end
         th_blink_10: begin
@@ -145,4 +145,4 @@ def test():
 
     assert(expected_code == code)
 
-test()
+test()