templates: Clean some whitespace issues in markdown_help.

Unfortunately, the rest cannot be easily fixed.

templates/zerver/markdown_help.html

@@ -3,10 +3,12 @@
     <div class="modal-body" tabindex="0">
         <div id="markdown-instructions">
             <table class="table table-striped table-condensed table-rounded table-bordered help-table">
-                <thead><tr>
+                <thead>
+                    <tr>
                         <th>{% trans %}You type{% endtrans %}</th>
                         <th>{% trans %}You get{% endtrans %}</th>
-                </tr></thead>
+                    </tr>
+                </thead>
 
                 <tbody>
                     <tr>

tools/check-templates

@@ -100,6 +100,7 @@ def check_html_templates(templates, all_dups):
             'templates/zerver/features.html',
             'templates/zerver/home.html',
             'templates/zerver/login.html',
+            # Can't clean this because of `preserve_spaces`
             'templates/zerver/markdown_help.html',
             'zerver/webhooks/taiga/doc.html',
             'zerver/webhooks/zendesk/doc.html',