Fix Gazebo warning for non-unique names in URDF

[oscar-lima]

This PR resolves the following Gazebo warning:

Warning [Model.cc:216] Non-unique name[mobipick/ft300_mounting_plate] detected 2 times in XML children of model with name[mobipick].

Cause:
The URDF file had a joint and link both named ft300_mounting_plate, causing a non-critical but undesired warning.

Solution:
Renamed the joint from ft300_mounting_plate to ft300_mounting_plate_joint to ensure name uniqueness.