t2500: fix test case for orphan request notifications

Since commit a66c7fa6

 (notify.py: Use a/an correctly when sending request
notifications, 2019-08-09), the body of notification emails sent when
filing orphan requests refers to "an orphan request" instead of "a
orphan request".
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

test/t2500-notify.sh

@@ -289,7 +289,7 @@ test_expect_success 'Test subject and body of request open notifications.' '
 	sed -n "/^\$/,\$p" sendmail.out | base64 -d >actual &&
 	echo >>actual &&
 	cat <<-EOD >expected &&
-	user [1] filed a orphan request for foobar [2]:
+	user [1] filed an orphan request for foobar [2]:
 
 	This is a request test comment.