docs: fix table in comparing observables guide (#22485)

PR Close #22485

aio/content/guide/comparing-observables.md

@@ -89,11 +89,14 @@ promise.then(() => {
 The following code snippets illustrate how the same kind of operation is defined using observables and promises.
 
 <table>
+  <thead>
     <tr>
       <th>Operation</th>
       <th>Observable</th>
       <th>Promise</th>
     </tr>
+  </thead>
+  <tbody>
     <tr>
       <td>Creation</td>
       <td>
@@ -130,6 +133,7 @@ The following code snippets illustrate how the same kind of operation is defined
       <td><pre>sub.unsubscribe();</pre></td>
       <td>Implied by promise resolution.</td>
     </tr>
+  </tbody>
 </table>
 
 ## Observables compared to events API