Add help for mregs

debugger/debugger.cpp

@@ -77,6 +77,7 @@ static void show_help() {
     cout << "  until X      -- execute until address X is reached" << endl;
     cout << "  go           -- exit debugger and continue emulator execution" << endl;
     cout << "  regs         -- dump content of the GRPs" << endl;
+    cout << "  mregs        -- dump content of the MMU registers" << endl;
     cout << "  set R=X      -- assign value X to register R" << endl;
     cout << "                  if R=loglevel, set the internal" << endl;
     cout << "                  log level to X whose range is -2...9" << endl;