docs: update config.yaml.example about upstream's keepalive.

[moonming]

Description

Fixes # (issue)

Checklist

• [ ] I have explained the need for this PR and the problem it solves
• [ ] I have explained the changes or the new features added to this PR
• [ ] I have added tests corresponding to this change
• [ ] I have updated the documentation to reflect this change
• [ ] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)