fix: removed string concat in a loop is inefficient warning

[corbinlazarone]

Summary

New users following the tutorial get "using string += string in a loop is inefficient" from the go lsp. I removed this warning by replacing the string concatenation with strings.builder go package.

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

[themavik]

Reviewed the changes — the approach looks correct and follows the existing patterns in the codebase.

[themavik]

Builder change is fine; this could still trip staticcheck if other examples/tutorials concatenate in a loop the same way, so worth grepping for the same pattern while you are here.