documentation: update osd pool default size from 2 to 3.

Fixes: #7725

Signed-off-by: Florent Flament <florent.flament@cloudwatt.com>
Merge pull request #1851 from FlorentFlament/master

documentation: update osd pool default size from 2 to 3.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>