get.json documents deprecated _source_include (#38613)

This brings it inline with other REST spec files which only document the plural

rest-api-spec/src/main/resources/rest-api-spec/api/get.json

@@ -65,14 +65,6 @@
           "type" : "list",
           "description" : "A list of fields to extract and return from the _source field"
         },
-        "_source_exclude": {
-          "type" : "list",
-          "description" : "A list of fields to exclude from the returned _source field"
-        },
-        "_source_include": {
-          "type" : "list",
-          "description" : "A list of fields to extract and return from the _source field"
-        },
         "version" : {
           "type" : "number",
           "description" : "Explicit version number for concurrency control"