update_engine: Correct test command in README.md

BUG=None
TEST=cros_run_unit_tests --board ${BOARD} --packages update_engine

Change-Id: I51552c7ca7b3155db6dad846b80f95c403815025
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2122354
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
diff --git a/README.md b/README.md
index 82642f7..71f271b 100644
--- a/README.md
+++ b/README.md
@@ -478,7 +478,7 @@
 or
 
 ```bash
-(chroot) $ cros_run_unit_tests --board ${BOARD} --test --packages update_engine
+(chroot) $ cros_run_unit_tests --board ${BOARD} --packages update_engine
 ```
 
 The above commands run all the unit tests, but `update_engine` package is quite