Fix a typo in pg_rotate_logfile

Hi,

I happened to notice a typo in pg_rotate_logfile in ipc/signalfuncs.c
- the hint message wrongly mentions that pg_logfile_rotate is part of
the core; which is actually not. pg_logfile_rotate is an adminpack's
1.0 SQL function dropped in 2.0. The core defines pg_rotate_logfile
SQL function instead, so use that. Here's a patch to fix the typo.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com