users-guide: Fix formatting of eventlog format documentation
users-guide: Fix formatting of eventlog format documentation

Test Plan: Read it

Reviewers: mpickering

Reviewed By: mpickering

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5264

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc