Fixes bug in top bottom pattern settings

As discussed in CCB, mistake was made which led to no skin being printed. This fixes it.
PP-44

resources/definitions/ultimaker3.def.json

@@ -174,7 +174,7 @@
         "support_z_distance": { "value": "0" },
         "switch_extruder_prime_speed": { "value": "15" },
         "switch_extruder_retraction_amount": { "value": "8" },
-        "top_bottom_pattern": { "value": "zigzag" },
+        "top_bottom_pattern": { "value": "'zigzag'" },
         "top_bottom_thickness": { "value": "1" },
         "travel_avoid_distance": { "value": "3 if extruders_enabled_count > 1 else machine_nozzle_tip_outer_diameter / 2 * 1.5" },
         "wall_0_inset": { "value": "0" },

resources/definitions/ultimaker_s3.def.json

@@ -159,7 +159,7 @@
         "support_z_distance": { "value": "0" },
         "switch_extruder_prime_speed": { "value": "15" },
         "switch_extruder_retraction_amount": { "value": "8" },
-        "top_bottom_pattern": { "value": "zigzag" },
+        "top_bottom_pattern": { "value": "'zigzag'" },
         "top_bottom_thickness": { "value": "1" },
         "travel_avoid_supports": { "value": "True" },
         "travel_avoid_distance": { "value": "3 if extruders_enabled_count > 1 else machine_nozzle_tip_outer_diameter / 2 * 1.5" },

resources/definitions/ultimaker_s5.def.json

@@ -163,7 +163,7 @@
         "support_z_distance": { "value": "0" },
         "switch_extruder_prime_speed": { "value": "15" },
         "switch_extruder_retraction_amount": { "value": "8" },
-        "top_bottom_pattern": { "value": "zigzag" },
+        "top_bottom_pattern": { "value": "'zigzag'" },
         "top_bottom_thickness": { "value": "1" },
         "travel_avoid_supports": { "value": "True" },
         "travel_avoid_distance": { "value": "3 if extruders_enabled_count > 1 else machine_nozzle_tip_outer_diameter / 2 * 1.5" },