Update slip-0015.md

update highlight (json->javascript) to hide red error

slip-0015.md

@@ -130,7 +130,7 @@ All labels can have any unicode letters. Empty string is treated in the software
 
 An example object looks like this:
 
-``` json
+``` javascript
 {
   "version": "1.0.0",
   "accountLabel": "Saving account", // one file per account, so only 1 label needed