docs(reactive-forms): fix one missed el in the example

Author: kapunahelewong

public/docs/_examples/reactive-forms/ts/src/app/hero-detail-6.component.html

@@ -30,7 +30,7 @@ <h3><i>PatchValue to initialize a value</i></h3>
     </label>
   </div>
   <div class="form-group radio">
-    <label>Super power:</label>
+    <h4>Super power:</h4>
     <label class="center-block"><input type="radio" formControlName="power" value="flight">Flight</label>
     <label class="center-block"><input type="radio" formControlName="power" value="x-ray vision">X-ray vision</label>
     <label class="center-block"><input type="radio" formControlName="power" value="strength">Strength</label>