docs: Fix environment variable DATABASE_URL to DSN

[sawadashota]

Proposed changes

Deprecated DATABASE_URL and replaced dsn at #1314
Now, warning log appears when serve with DATABASE_URL.

$ DATABASE_URL=memory go run main.go serve all
...
WARN[0000] Configuration key DATABASE_URL is deprecated and will be removed in a future release. Use key dsn instead!

Fix DSN, then no warnings appeared.

$ DSN=memory go run main.go serve all

So that I just fixed docs DATABASE_URL to DSN.

Checklist

• I have read the contributing guidelines
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I signed the Developer's Certificate of Origin
  by signing my commit(s). You can amend your signature to the most recent commit by using git commit --amend -s. If you
  amend the commit, you might need to force push using git push --force HEAD:<branch>. Please be very careful when using
  force push.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

[aeneasr]

Thank you!