serializers: bibtex: Conference proceeding to proceedings

[ptamarit]

❤️ Thank you for your contribution!

Description

• Fixes Resource type "Publication / Conference proceeding" should serialize to BibTeX type "proceedings" #1914
• See description in linked issue

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[tmorrell]

Ah, you already did this. Great!

[ptamarit]

Just reformatted the code to appear on one line.