Minor updates to Three Bridges map

A minor update to the three bridges map. The geometry is the same, but it has incorporated bugfixes from the latest release of UrdfSim (Contains build from commit a78e6301220fbc97724fd742ceb5ec04270f638d)

Initial Release of Three Bridges map

This release contains the following maps:

• Three Bridges

With this release, the maps page has been updated to contain information about the contents of each map and how to use each map.