Fixed invalid cluster ore config (#517)

map_gen/Diggy/Orepattern/Clusters.lua

@@ -20,7 +20,7 @@
 --   simplex: similar to perlin
 --   zero: does nothing with this source
 --   one: adds the weight directly to the noise calculation
-clusters = {
+return {
     {
         yield=1.0,
         min_distance=30,
@@ -48,4 +48,4 @@ clusters = {
             ['uranium-ore'] = 86,
             ['crude-oil']   = 57,
         }, },
-}
+}