README.md: correct postgresql_conn_validator example

[bastelfreak]

it's required to set the psql_path parameter. Otherwise you end up with
something like:

Debug: Executing: '--tuples-only --quiet --no-psqlrc --host 127.0.0.1
--port 5432 --username grafana --dbname grafana --command SELECT 1'

[smortex]

A valid default value would be better, but at least show that it does not exist! LGTM!

[bastelfreak]

@smortex I totally agree. I was really surprised to find out that there isn't. Maybe there was one in the past 🤔

[ekohl]

Doesn't look like it ever had a default.
c37b50c
I wouldn't mind one, or it could be modified to use $PATH by default.

[github-actions]

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[kenyon]

Bad @github-actions bot.

[david22swan]

@bastelfreak Apologies on the wait but this LGTM so have gone ahead and merged.
Thank's for your contribution :)