doc: fix hint on how to run bt_navigator tests with ctest.

Use -R to invoke the regular expresion.

Signed-off-by: Mike Wake <[email protected]>

nav2_system_tests/src/system/README.md

@@ -1,6 +1,6 @@
 # Nav2 System Tests
 
-This is a 'top level' system test which will use Gazebo to simulate a Robot moving from an known initial starting position to a goal pose. 
+This is a 'top level' system test which will use Gazebo to simulate a Robot moving from an known initial starting position to a goal pose.
 
 ## To run the test
 First, you must build Nav2 including this package:
@@ -16,7 +16,7 @@ Output results will go to the screen, and will be logged to the "log/latest_test
 To run just the bt_navigator test:
 ```
 cd build/nav2_system_tests
-ctest -V bt_navigator$
+ctest -V -R bt_navigator$
 ```
 
 To loop over the bt_navigator test, a script has been provided: