docs(pgxpool): correct the documented pool_max_conns default

[shaikn6]

ParseConfig's doc comment lists:

//   - pool_max_conns: integer greater than 0 (default 4)

But when pool_max_conns is not supplied, the code sets it to the greater of 4 and runtime.NumCPU():

} else {
    config.MaxConns = defaultMaxConns            // 4
    if numCPU := int32(runtime.NumCPU()); numCPU > config.MaxConns {
        config.MaxConns = numCPU
    }
}

So on any host with more than 4 CPUs the effective default is NumCPU, not 4 — which matters for capacity planning against max_connections. The Config.MaxConns field doc already states this correctly ("The default is the greater of 4 or runtime.NumCPU()"); this just brings the ParseConfig parameter list in line with it.

One-line comment change. No code, no behavior change.