content: update balance.go to remove compile errors

Explicitly cast time.Duration values to avoid compile-time errors.

Change-Id: I394c248704023234c6e54bcde696734a35d4b5d0
GitHub-Last-Rev: 252a12f6a028d43ac0139ec99e52b20ca521c5d7
GitHub-Pull-Request: golang/talks#4
Reviewed-on: https://go-review.googlesource.com/c/talks/+/184083
Reviewed-by: Andrew Bonventre <andybons@golang.org>
1 file changed