Fix typo

src/ubuntu.ipxe

@@ -11,7 +11,7 @@ clear ubuntu_version
 set ubuntu_mirror ${mirror}
 menu ${os} - ${arch_a} - Image Sig Checks: [${img_sigs_enabled}]
 item --gap Latest Releases
-item yakkety ${space ${os} 16.10 Yakkety Yak
+item yakkety ${space} ${os} 16.10 Yakkety Yak
 item xenial ${space} ${os} 16.04 LTS Xenial Xerus
 item wily ${space} ${os} 15.10 Wily Werewolf
 item vivid ${space} ${os} 15.04 Vivid Vervet