patch 8.0.0180: error E937 is used twice

Problem:    Error E937 is used both for duplicate key in JSON and for trying
            to delete a buffer that is in use.
Solution:   Rename the JSON error to E938. (Norio Takagi, closes #1376)
diff --git a/src/json.c b/src/json.c
index faaeab3..a9333cf 100644
--- a/src/json.c
+++ b/src/json.c
@@ -936,7 +936,7 @@
 			&& dict_find(top_item->jd_tv.vval.v_dict,
 						 top_item->jd_key, -1) != NULL)
 		{
-		    EMSG2(_("E937: Duplicate key in JSON: \"%s\""),
+		    EMSG2(_("E938: Duplicate key in JSON: \"%s\""),
 							     top_item->jd_key);
 		    clear_tv(&top_item->jd_key_tv);
 		    clear_tv(cur_item);