[DOCS] Added explicit "lang" field to documentation of script score definition

docs/reference/modules/scripting.asciidoc

@@ -126,6 +126,7 @@ curl -XPOST localhost:9200/_search -d '{
       "functions": [
         {
           "script_score": {
+            "lang": "groovy",
             "script_file": "calculate-score",
             "params": {
               "my_modifier": 8