generate-self-signed-cert: Fix a mostly-harmless bug.

Thanks to the magic of `set -x`, I noticed this:
```
+ cat
++ ssl-cert
/tmp/src/zulip-server/scripts/setup/generate-self-signed-cert: line 49: ssl-cert: command not found
+ apt-get install -y openssl
[...]
```

In other words, we were trying to run `ssl-cert` -- the name of a
Debian package I meant to refer to in a comment inside the templated
temporary config file for `openssl req` -- as if it were a command.
It wasn't, hence the error.

Because `set -e` has loopholes like a sieve, this didn't cause the
script to exit, just produced this funny output and presumably caused
the config file's comment to be missing a word.  In principle, it
could do something surprising if for some reason there were a command
named `ssl-cert` on PATH.

Fix it.

scripts/setup/generate-self-signed-cert

@@ -47,7 +47,7 @@ config="$(mktemp)" || exit 1
 trap 'rm -f "$config"' EXIT
 
 cat >"$config" <<EOF
-# Based on /usr/share/ssl-cert/ssleay.cnf from Debian's `ssl-cert`
+# Based on /usr/share/ssl-cert/ssleay.cnf from Debian's \`ssl-cert\`
 # package, which is used for the system's snakeoil cert in /etc/ssl/.
 
 RANDFILE            = /dev/urandom