Fix broken block code in Bracket notation section (mdn#18507)

Add three missing backticks at ending of block code

files/en-us/learn/javascript/objects/basics/index.md

@@ -188,6 +188,7 @@ const person = {
 }
 const input = prompt('Get name or age?')
 console.log(person[input])
+```
 
 ## Setting object members