Changed name of terrain model to prevent conflict (space-ros#46)

Signed-off-by: Jasmeet Singh <[email protected]>

Author: jasmeet0915

Diff for: mars_rover/worlds/mars_curiosity.world

@@ -1,8 +1,5 @@
 <?xml version="1.0" ?>
 <sdf version="1.11">
-  <world name="default">
-<?xml version="1.0" ?>
-<sdf version="1.6">
   <world name="default">
     <physics name="1ms" type="ignored">
       <max_step_size>0.001</max_step_size>