pgsql: Fix vcregress.pl's ancient misspelling of --max-connections.

Fix vcregress.pl's ancient misspelling of --max-connections.

I copied the existing spelling of "--max_connections", but
that's just wrong :-(. Evidently setting $ENV{MAX_CONNECTIONS}
has never actually worked in this script. Given the lack of
complaints, it's probably not worth back-patching a fix.

Per buildfarm.

Discussion: /messages/by-id/899209.1620759506@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0b85fa93e4575183aa5a71ebe3c6bae8d97704ed

Modified Files
--------------
src/tools/msvc/vcregress.pl | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)