Explicitly convert display name to string

An React Elements displayName is not necessarily a string and Symbols are not automatically converted to strings.

Author: FabianSellmann

src/formatter/formatReactElementNode.js

@@ -203,7 +203,7 @@ export default (
       out += '\n';
       out += spacer(newLvl - 1, tabStop);
     }
-    out += `</${displayName}>`;
+    out += `</${displayName.toString()}>`;
   } else {
     if (
       !isInlineAttributeTooLong(