[FIX] documentation: typical command to launch odoo is not usable

The typical command to launch odoo in the install documentation is usable as it is after following the complete instructions.
There is two changes done here:
- Remove the ../my_modules from the command, as it is a directory not used in the page at all.
- Remove the db-filter option not mandatory since the 11.0 version and add a '-d' argument. This will create the db instead of showing the database selector to the reader.

closes odoo/odoo#43653

Signed-off-by: Florian Daloze (fda) <fda@odoo.com>