Fix PercolateDescriptor sort serialization

When serializing the PercolateDescriptor, the sort property was
incorrectly being serialized as 'score' and not 'sort'. This is now
fixed

Author: mikelazell

Diff for: src/Nest/DSL/MultiPercolate/IPercolateOperation.cs

@@ -18,7 +18,7 @@ public interface IPercolateOperation
 		[JsonProperty(PropertyName = "track_scores")]
 		bool? TrackScores { get; set; }
 
-		[JsonProperty(PropertyName = "score")]
+		[JsonProperty(PropertyName = "sort")]
 		[JsonConverter(typeof(DictionaryKeysAreNotPropertyNamesJsonConverter))]
 		IDictionary<PropertyPathMarker, ISort> Sort { get; set; }