Minor fix for navsat_transform launch param

[mabelzhang]

(I swear I didn't break this one with my PRs. File was last modified 2 months ago.)

Minor fix to appease rclcpp parameter complaining about a double being an int.

Without the fix, navsat_transform.launch.py cannot be launched:

$ ros2 launch robot_localization navsat_transform.launch.py 
...
[navsat_transform_node-1] terminate called after throwing an instance of 'rclcpp::ParameterTypeException'
[navsat_transform_node-1]   what():  expected [double] got [integer]
[ERROR] [navsat_transform_node-1]: process has died [pid 8783, exit code -6, cmd '/home/developer/tri_open_source/colcon_ws_debug/install/robot_localization/lib/robot_localization/navsat_transform_node __node:=navsat_transform_node __params:=/home/developer/tri_open_source/colcon_ws_debug/install/robot_localization/share/robot_localization/params/navsat_transform.yaml'].

On a related note, a ROS 2 ticket exists to make integer acceptable when double is expected integer to double ros2/rclcpp#979