Fix missing break in last example (#2629)

In the last chunk of code, it was missing the `break` (or `return`). So, if used as is, the bold key will toggle the `code` feature as well.

docs/walkthroughs/applying-custom-formatting.md

@@ -146,11 +146,13 @@ class App extends React.Component {
       case 'b': {
         event.preventDefault()
         editor.toggleMark('bold')
+        break
       }
       case '`': {
         const isCode = editor.value.blocks.some(block => block.type == 'code')
         event.preventDefault()
         editor.setBlocks(isCode ? 'paragraph' : 'code')
+        break
       }
       default: {
         return next()