www: drop --subject from "git send-email" instructions
Apparently, --subject doesn't work[1] with "git send-email" in
this context. So drop the CLI arg and add a note to tell the
user to set a "Subject:" line in their response body, instead.
[1] I'm not sure if --subject ever worked as I thought it would,
or if it's a regression. In either case, there are current
versions of git where it doesn't, so just tell users to use
the currently supported method.
Link: https://80x24.org/lore/git/CAC4O8c-Tf11CpwuRudyrpXv5bGshuyEenV9kKrs0zRWER-+yHA@mail.gmail.com/