fix: remove invalid character for XML feed; fix #537

[geekplux]

Summary

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI/CSS related code, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

close #537

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[egoist]

I opened an issue to see if it's possible to be fixed by the upstream https://github.com/jpmonette/feed/issues/107