qapi: Fix a misleading parser error message

When choking on a token where an expression is expected, we report
'Expected "{", "[" or string'.  Close, but no cigar.  Fix it to
Expected '"{", "[", string, boolean or "null"'.

Missed in commit e53188ad.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-48-git-send-email-armbru@redhat.com>