Fix the nodeName from lower to upper case (#5348)

The node name should be `STRONG` instead of `strong` in the example deserialize function.

docs/concepts/10-serializing.md

@@ -173,7 +173,7 @@ const deserialize = (el, markAttributes = {}) => {
 
   // define attributes for text nodes
   switch (el.nodeName) {
-    case 'strong':
+    case 'STRONG':
       nodeAttributes.bold = true
   }