Update README.md (bokysan#61)

Fix configuration of POSTFIX_myhostname

README.md

@@ -263,15 +263,15 @@ Example:
 docker run --rm --name postfix -e "SMTP_HEADER_CHECKS="regexp:/etc/postfix/smtp_header_checks" -e "ALLOWED_SENDER_DOMAINS=example.com example.org" -p 1587:587 boky/postfix
 ```
 
-#### `POSTFIX_hostname`
+#### `POSTFIX_myhostname`
 
 You may configure a specific hostname that the SMTP server will use to identify itself. If you don't do it,
 the default Docker host name will be used. A lot of times, this will be just the container id (e.g. `f73792d540a5`)
 which may make it difficult to track your emails in the log files. If you care about tracking at all,
 I suggest you set this variable, e.g.:
 
 ```shell script
-docker run --rm --name postfix -e "POSTFIX_hostname=postfix-docker" -p 1587:587 boky/postfix
+docker run --rm --name postfix -e "POSTFIX_myhostname=postfix-docker" -p 1587:587 boky/postfix
 ```
 
 #### `POSTFIX_mynetworks`