Doc: fix ancient mistake in search_path documentation.
"$user" in a search_path string is replaced by CURRENT_USER not
SESSION_USER. (It actually was SESSION_USER in the initial implementation,
but we changed it shortly later, and evidently forgot to fix the docs to
match.)
Noted by antonov@stdpr.ru
Discussion: https://postgr.es/m/
159151fb45d490c8d31ea9707e9ba99d@stdpr.ru