[FIX] crm: improve layout of conversion modal

When converting a lead to an opportunity and merging with existing
opportunities, the list view showing opportunities is very tiny.

This is caused by the field not having the correct colspan, as its label is
hidden with the "nolabel" attribute.

This commit fixes it by assigning a colpan of 2.

Task-3179173

closes odoo/odoo#119215

Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>