Update lex.go (#1517)

Fixing printing error, printing rune instead of integer

## Buildtools PR checklist

- [x] The code in this PR is covered by unit/integration tests.
- [ ] I have tested these changes and provide testing instructions
below.
- [ ] I have either responded to, or resolved all Gemini comments on the
PR.
- [x] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
reasoning for why they can not be followed.

## Description

What does this PR do? Please follow [Google Eng Practices -
Description](https://google.github.io/eng-practices/review/developer/cl-descriptions.html)

### (optional) These changes were tested using the following steps

Steps to reproduce / test the PR changes (if anything besides unit
tests).