Update cmf_sandbox.rst

Adding "php app/console doctrine:phpcr:init:dbal" to setup the database using pdo_mysql otherwise I have this error:

<pre>
An exception occurred while executing 'SELECT * FROM phpcr_namespaces':
SQLSTATE[42S02]: Base table or view not found: 1146 Table 'cmf.phpcr_namespaces' doesn't exist
Executing initializer: CmfRoutingBundle
</pre>