docs: formatdomain: Tweak disk discard= docs

Change from

  'controls whether to discard ... requests are ignored'

to

  'controls whether discard requests ... are ignored'

docs/formatdomain.html.in

@@ -3002,7 +3002,7 @@
           </li>
           <li>
             The optional <code>discard</code> attribute controls whether
-            to discard (also known as "trim" or "unmap") requests are
+            discard requests (also known as "trim" or "unmap") are
             ignored or passed to the filesystem. The value can be either
             "unmap" (allow the discard request to be passed) or "ignore"
             (ignore the discard request).