doc: Fix pg_buffercache_evict() title

doc: Fix pg_buffercache_evict() title

Use <function> rather than <structname> in the <title> to be consistent
with how other functions in this module are documented. Also suffix the
function name with () for consistency.

Backpatch to v17 where pg_buffercache_evict was introduced.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAExHW5uKWH8CuZc9NCb8XxSQc6uzvACV0cScebm54kF763ERAw@mail.gmail.com
Backpatch-through: 17