Clarify username for postgresql's peer authentication method

[mario-campos]

The documentation is not clear that the database user name should be the name of the PHP process when using PostgreSQL's peer authentication method. This was only hinted at in a subsequent paragraph, after I had run the PSQL commands with the user name username.

I also updated the example to use the common www-data user name, which will probably familiar to most users.

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• Fix #…

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