templates: Fix new low-hanging HTML validation problems.

"http://localhost:9981/api/update-message-flags":318.16-319.34: error: “th” start tag in table body.
"http://localhost:9981/api/update-message-flags":344.35-344.38: error: Stray end tag “a”.
"http://localhost:9981/api/incoming-webhooks-walkthrough":412.4-412.77: error: An “img” element must have an “alt” attribute, except under certain conditions. For details, consult guidance on providing text alternatives for images.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

templates/zerver/api/incoming-webhooks-walkthrough.md

@@ -269,7 +269,7 @@ Feel free to use 4-spaces as tabs for indentation if you'd like!
 
 Your sample notification may look like:
 
-<img class="screenshot" src="/static/images/api/helloworld-webhook.png" />
+<img class="screenshot" src="/static/images/api/helloworld-webhook.png" alt="screenshot" />
 
 
 

templates/zerver/api/update-message-flags.md

@@ -62,8 +62,10 @@ curl -X POST {{ api_url }}/v1/messages/flags \
 <div>
     <table>
         <thead>
-            <th style="width:30%">Flag</th>
-            <th style="width:70%">Purpose</th>
+            <tr>
+                <th style="width:30%">Flag</th>
+                <th style="width:70%">Purpose</th>
+            </tr>
         </thead>
         <tbody>
             <tr>
@@ -87,7 +89,7 @@ curl -X POST {{ api_url }}/v1/messages/flags \
                 <td>`mentioned`</td>
                 <td>
                      Whether the current user [was
-                     mentioned](/help/mention-a-user-or-group)</a> by
+                     mentioned](/help/mention-a-user-or-group) by
                      this message, either directly or via a user
                      group.  Not editable.
                 </td>