[Template] Improve description of parameter ImageBuilderSubnetId.

Changes

Improve description of parameter ImageBuilderSubnetId.

[ ] I added tests to new or existing code
[ ] I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
[ ] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
[ ] I made sure that any GitHub issue solved by this PR is correctly linked
[ ] I checked that infrastructure/update_infrastructure.sh runs without any error
[ ] I checked that npm run build builds without any error
[ ] I checked that clusters are listed correctly
[ ] I checked that a new cluster can be created (config is produced and dry run passes)
[ ] I checked that login and logout work as expected

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