Update the typo in firststeps-rp.md

it should be HTTPS_BIND instead of HTTPS_PORT

docs/firststeps-rp.md

@@ -5,7 +5,7 @@ mailcow: dockerized trusts the default gateway IP 172.22.1.1 as proxy. This is v
 ``` bash
 HTTP_BIND=127.0.0.1
 HTTP_PORT=8080
-HTTPS_PORT=127.0.0.1
+HTTPS_BIND=127.0.0.1
 HTTPS_PORT=8443
 ```
 ** IMPORTANT: Do not use port 8081 **
@@ -77,4 +77,4 @@ backend mailcow
   http-request set-header X-Forwarded-Proto https if { ssl_fc }
   http-request set-header X-Forwarded-Proto http if !{ ssl_fc }
   server mailcow 127.0.0.1:8080 check
-```
+```