DOC/BUG: Fix documentation for `infer_datetime_format` #6073
Fix formatting typos and ensure the "foo.csv" ipython processing works.

Fixes #6073

@jorisvandenbossche @y-p ?
 Checking.
 Thanks! Looking good
 @y-p Ah, didn't see your comment, but I think it's OK
 no problem.
 That was fast.  Thanks guys.  Two minutes to spare on my 30 minute inflight internet pass!
