top.1: Fix a typo in description of H interactive command

B is listed as an interactive command to toggle the display of threads.
This is a typo introduced during the conversion of the manual page to
mdoc.

Fixes: 9d6cce02a78c967e69b603d503545c4b43d7765f
MFC after: 3 days

(cherry picked from commit b8135ed67ce86e40fa7b46653ff54c769f2e7c98)