imap-send: fix confusing 'store' terminology in error message

imap-send: fix confusing 'store' terminology in error message

The error message 'no imap store specified' is misleading because
it refers to 'store' when the actual missing configuration is
'imap.folder'. Update the message to use the correct terminology
that matches the configuration variable name.

This reduces confusion for users who might otherwise look for
non-existent 'imap.store' configuration when they see this error.

Signed-off-by: Jörg Thalheim <joerg@thalheim.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>