[FIX] mail, *: fix grammar error in field help

This commit fixes a really old grammar error in the help message of the
message_needaction_counter field.

Before this commit: “Number of messages which requires an action”
After: “Number of messages requiring action”

The subject of “require” is “messages”, which is third-person plural, so
it can't take the -s suffix.

closes odoo/odoo#128929

Related: odoo/enterprise#44299
Signed-off-by: Louis Wicket (wil) <wil@odoo.com>