Add comment about password for the ISPRS Potsdam dataset

[ameier3]

As mentioned by @AdeelH here I added a comment on where to find the password in the master branch

Overview

Brief description of what this PR does, and why it is needed.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• [x ] Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

Optional. Ancillary topics, caveats, alternative strategies that didn't work out, anything else.

Testing Instructions

• How to test this PR
• Prefer bulleted description
• Start after checking out this branch
• Include any setup required, such as rebuilding the Docker image.
• Include test case, and expected output if not captured by automated tests.

Closes #XXX

[AdeelH]

Thank you!