fix: Grammar of error statement in 404 page (#15447)

Co-authored-by: Suraj Shetty <13928957+surajshetty3416@users.noreply.github.com>

frappe/www/404.html

@@ -15,10 +15,10 @@
 			{{ _("There's nothing here") }}
 		</h2>
 		<div class="text-muted error-text">
-			{{ _("The page you are looking for have gone missing.") }}
+			{{ _("The page you are looking for has gone missing.") }}
 		</div>
 		<div class="mt-6 back-to-home"><a href='/' class='btn btn-primary'>{{ _("Back to Home") }}</a></div>
 	</div>
 </div>
 
-{% endblock %}
+{% endblock %}