Update bas-external-system.mdx with .env configuration

[drvup]

I did run the 2nd time into this small tiny and time-consuming issue. So, for the next time, everyone should be aware of it :)

What Has Changed?

The .env file needs to have the variables within 1 line, without breaks.

[marikaner]

Hey @drvup, it seems this was missed for quite a while. I used to think that .env only accepts values on one line, however, I just learned last week, that this is not the case. I believe our example is incorrect though, because it uses " for the inner strings and the outer string. I think this should also work:

destinations='[{
"name": "<destinationName>",
"url": "http://<destinationName>.dest",
"proxyConfiguration":{
      "host": "127.0.0.1",
      "port": 8887,
      "protocol":"http"
  }
}]'

I would update the example to reflect that instead. Any objections?