imap.h: Corrected incorrect comment clarification

Corrected comment clarification made in commit 167717b8.

lib/imap.h

@@ -55,7 +55,7 @@ typedef enum {
    struct */
 struct imap_conn {
   struct pingpong pp;
-  char *mailbox;          /* Message ID to fetch */
+  char *mailbox;          /* Mailbox to select */
   unsigned int authmechs; /* Accepted authentication mechanisms */
   unsigned int authused;  /* Auth mechanism used for the connection */
   imapstate state;        /* Always use imap.c:state() to change state! */