Change Postgres connection string example

[iJackUA]

Description

The provided example string was not working for me with Postgres.
I propose one that works.

Motivation and Context

It helps to start running Flagr with Postgres quickly

How Has This Been Tested?

Tested manually on local machine in docker compose env

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #296 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #296   +/-   ##
======================================
  Coverage    81.8%   81.8%           
======================================
  Files          26      26           
  Lines        1533    1533           
======================================
  Hits         1254    1254           
  Misses        210     210           
  Partials       69      69

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[zhouzhuojie]

LGTM, I think I also like the url style for consistency.

Both of the styles should work, and they are documented here: https://godoc.org/github.com/lib/pq