Fix imap.openBox not returning with highestmodseq`

Hello,

I noticed that when using `imap.openBox` the box in the response would not contain a highestmodseq despite the imap server sending it down. 

The problem was the server message `'* OK [HIGHESTMODSEQ 501328]'` not matching the RE_UNTAGGED regex correctly. The entirety of the [HIGHESTMODSEQ 501328] was being put into capture group 5 instead of 4 as the other commands do. This caused the message to be ignored by node-imap.

This does not happen on `imap.status`. The change just tweaks the regex, hopefully with no side effects.