Better default values for Mesh Inset

Marlin/src/config/examples/Formbot/T-Rex_2+/Configuration.h

   #define MESH_EDIT_GFX_OVERLAY      // Display a graphics overlay while editing the mesh
 
   #ifdef ROXYs_TRex
-    #define MESH_INSET 25            // Set Mesh bounds as an inset region of the bed
+    #define MESH_INSET 35            // Set Mesh bounds as an inset region of the bed
   #else
     #define MESH_INSET 0
   #endif