[lib] Fix typo assetMediaMessageType => assertMediaMessageType

Summary: Super minor fix.

Test Plan: This function isn't used anywhere in the codebase. I'm about to use it though.

Reviewers: tomek, ginsu, rohan, marcin, kamil, O2 Blocking Reviewers

Reviewed By: tomek, rohan, O2 Blocking Reviewers

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5635