Fix BetterNether's Bone Spores not giving Bone Meal

Addresses the bug reported in #368. The spores now have a 15% chance to
drop 1-2 bone meal which is a similar rate to the secondary products of
other crops like carrots.

common/src/main/resources/data/botanypots/recipes/betternether/crop/bone_mushroom.json

@@ -25,6 +25,14 @@
       "output": {
         "item": "betternether:bone_mushroom"
       }
+    },
+    {
+      "chance": 0.15,
+      "output": {
+        "item": "minecraft:bone_meal"
+      },
+      "minRolls": 1,
+      "maxRolls": 2
     }
   ]
 }