Correct default database prefix in documentation

Summary: Default value of storage.default-namespace is still phabricator. Correct documentation about this.

Test Plan: Compare documentation with src/applications/config/option/PhabricatorMySQLConfigOptions.php.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25463