INSTALL.md: Restore $ as command prompt indicator

INSTALL.md: Restore $ as command prompt indicator

We have a notational convention in INSTALL.md, which says this among
others:

> Any line starting with a dollar sign is a command line.
>
>     $ command
>
> The dollar sign indicates the shell prompt and is not to be entered as
> part of the command.

That notation exists to make it clear what is a command line and
what's output from that command line.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/12257)