docs: fix codegen tutorial path separators

[miachillgood]

Summary

• replace the Windows-style handler file path in the codegen tutorial with a forward-slash path
• keep the tutorial path style consistent with the surrounding commands and file references

Why

Addresses #1079. The tutorial currently mixes internal\handlers\pet.go with slash-based paths like cmd/Petdemo/main.go, which is confusing in a cross-platform docs page.

Testing

• not run (docs-only change)