Fix synonyms config for indexing (#2101)

Author: reakaleek

src/Elastic.Markdown/Exporters/Elasticsearch/ElasticsearchExporter.cs

@@ -164,8 +164,9 @@ protected static string CreateMappingSetting(string synonymSetName) =>
 		    "filter": {
 		      "synonyms_filter": {
 				  "type": "synonym_graph",
-				  "synonyms_set": "{{synonymSetName}}"
-				  },
+				  "synonyms_set": "{{synonymSetName}}",
+				  "updateable": true
+			  },
 		      "english_stop": {
 		        "type": "stop",
 		        "stopwords": "_english_"