fix: the docs website build failed due to incorrect unclosed tag <br>

[guoqqqi]

Description

This PR fixes build failures caused by unclosed HTML <br> tags in the opentelemetry.md documentation file.

The changes:

• update unclosed <br> tags to <br />

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)

[guoqqqi]

cc @Yilialinn PTAL