docs: Use proper article in formatdomain.html.in

It's "a hard_limit", not "an hard_limit".  Probably that was just a typo.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/formatdomain.html.in

@@ -1085,7 +1085,7 @@
         of memory, which means a malicious guest allocating large amounts of
         locked memory could cause a denial-of-service attack on the host.
         Because of this, using this option is discouraged unless your workload
-        demands it; even then, it's highly recommended to set an
+        demands it; even then, it's highly recommended to set a
         <code>hard_limit</code> (see
         <a href="#elementsMemoryTuning">memory tuning</a>) on memory allocation
         suitable for the specific environment at the same time to mitigate