Output a more realistic example in this message

Author: michaelklishin

Makefile

@@ -119,7 +119,7 @@ test:: $(ESCRIPTS)
 ifdef TEST_FILE
 	$(gen_verbose) $(MIX_TEST) $(TEST_FILE)
 else
-	$(verbose) echo "TEST_FILE must be set, e.g. TEST_FILE=./test/close_all_connections_command_test.exs" 1>&2; false
+	$(verbose) echo "TEST_FILE must be set, e.g. TEST_FILE=./test/ctl" 1>&2; false
 endif
 
 dialyzer:: $(ESCRIPTS)