Update contributing doc

[dentiny]

When I'm setting up the python dependency, I found existing command doesn't work on my end.

Error message is

[/workspaces/pg_duckdb] (hjiang/support-interval-type-2) 
postgres@dd042a1658a0$ pip3 -m venv

Usage:   
  pip3 <command> [options]

no such option: -m
[/workspaces/pg_duckdb] (hjiang/support-interval-type-2) 
postgres@dd042a1658a0$ pip -m venv

Usage:   
  pip <command> [options]

no such option: -m

Instead, python -m venv env seems to be the correct command

[/workspaces/pg_duckdb] (hjiang/support-interval-type-2) 
postgres@dd042a1658a0$ python3 -m venv env
[/workspaces/pg_duckdb] (hjiang/support-interval-type-2) 
postgres@dd042a1658a0$ echo $?
0

[JelteF]

I think this has simply just been always incorrect. Thanks!