Fixed bookstack env variable template

[akhfa]

I fixed environment variable needed by latest bookstack image version

☑️ Self Check before Merge

• I have tested the template using the method described in README.md thoroughly
• I have ensured that I put as much default values as possible (except passwords) to ensure minimum effort required for end users to get started.
• I have ensured that I am not using the "latest" tag as this tag is dynamically changing and might break the one-click app. Use a fixed version.
• I have made sure that instructions.start and instructions.end are clear and self-explanatory.
• Icon is added as a png file to the logos directory.
• I've executed the checks if necessary by running npm ci && npm run validate_apps && npm run formatter (If failling run the prettier: npm run formatter-write)
• I will take responsibility addressing any issues that arises as a result of this PR (maintaining this app).