Correct the use of connections strings in integration documents

[alistairmatthews]

Although in the Dashboard there is often an environment variable such as ConnectionStrings__postgresdb listed, this shouldn't be used in code for two reasons:

• The double-underscore returns null and should be replaced with a colon.
• We should always encourage the Aspire method instead: var connectionUri = builder.Configuration.GetConnectionString("postgresdb");

Fixes: #312