Update gazebo_sitl_multiple_run.sh (#15382)

If `make px4_sitl_default` is run, and not `make px4_sitl_default gazebo` then each model instance will throw the following errors:
```
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_gps_plugin.so: libgazebo_gps_plugin.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_multirotor_base_plugin.so: libgazebo_multirotor_base_plugin.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_motor_model.so: libgazebo_motor_model.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_motor_model.so: libgazebo_motor_model.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_motor_model.so: libgazebo_motor_model.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_motor_model.so: libgazebo_motor_model.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_groundtruth_plugin.so: libgazebo_groundtruth_plugin.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_magnetometer_plugin.so: libgazebo_magnetometer_plugin.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_barometer_plugin.so: libgazebo_barometer_plugin.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_mavlink_interface.so: libgazebo_mavlink_interface.so: cannot open shared object file: No such file or directory
[Err] [Plugin.hh:178] Failed to load plugin libgazebo_imu_plugin.so: libgazebo_imu_plugin.so: cannot open shared object file: No such file or directory
```

Tools/gazebo_sitl_multiple_run.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 # run multiple instances of the 'px4' binary, with the gazebo SITL simulation
-# It assumes px4 is already built, with 'make px4_sitl_default'
+# It assumes px4 is already built, with 'make px4_sitl_default gazebo'
 
 # The simulator is expected to send to TCP port 4560+i for i in [0, N-1]
 # For example gazebo can be run like this: