[help] remove link from about.md title

Now that about.html extends page_with_header.html
it already has a link to the start page and removing
the link makes it easier to extract the page title
from the Markdown for the following commit.

searx/help/about.md

@@ -1,4 +1,4 @@
-# About [searxng][url_for:index]
+# About SearXNG
 
 SearXNG is a fork from the well-known [searx] [metasearch engine], aggregating
 the results of other [search engines][url_for:preferences] while not storing

tests/unit/test_webapp.py

@@ -176,7 +176,7 @@ class ViewsTestCase(SearxTestCase):
     def test_about(self):
         result = self.app.get('/about')
         self.assertEqual(result.status_code, 200)
-        self.assertIn(b'<h1>About <a href="/">searxng</a></h1>', result.data)
+        self.assertIn(b'<h1>About SearXNG</h1>', result.data)
 
     def test_health(self):
         result = self.app.get('/healthz')