Add quotes to description of -A parameter

[huitema]

As seen in issue #1513, some users are confused by the syntax of the "-A" option. This PR tries to address the confusion by making the "usage" text in picoquicdemo more explicit:

• Use quotes around the example parameters, because many shells will apply special treatment to "," between addresses
• State explicitly that the "-A" parameter only applies to the client.

[clarkzjw]

Do we want to support extra whitespaces in the multipath config option? Because currently -A "10.0.2.2/2, 10.0.3.2/3" won't work in https://github.com/private-octopus/picoquic/blob/master/picoquicfirst/picoquicdemo.c#L444

[huitema]

@clarkzjw I think that we do not. The "usage" example do not include white spaces:

Picoquic demo options:
  -A "ip/ifindex[,ip/ifindex]"  IP and interface index for multipath alternative
                        path, e.g. "10.0.0.2/3,10.0.0.3/4". This option only
                        affects the behavior of the client.

Do you think that we should update the parsing code to support them?

[clarkzjw]

Considering the demonstration purpose of picoquicdemo, I think we can leave it as is for now, unless we see the necessity in the future.