Update KeepWordsTokenFilter.cs

Make the Json match the spec
https://www.elastic.co/guide/en/elasticsearch/reference/current/analysis-keep-words-tokenfilter.html

Author: davidtme

src/Nest/Domain/Analysis/TokenFilter/KeepWordsTokenFilter.cs

@@ -23,7 +23,7 @@ public KeepWordsTokenFilter()
 		/// <summary>
 		/// A path to a words file.
 		/// </summary>
-		[JsonProperty("rules_path")]
+		[JsonProperty("keep_words_path")]
 		public string KeepWordsPath { get; set; }
 
 		/// <summary>
@@ -33,4 +33,4 @@ public KeepWordsTokenFilter()
 		public bool? KeepWordsCase { get; set; }
 
 	}
-}
+}