This commit fixes the comment about SharedHotStandbyActive variable.
The comment was apparently copy-and-pasted.

Author: Atsushi Torikoshi
Discussion: https://postgr.es/m/CACZ0uYEjpqZB9wN2Rwc_RMvDybyYqdbkPuDr1NyxJg4f9yGfMw@mail.gmail.com