feat: adjust postgres-create-user.md

doc/postgres-create-user.md

@@ -7,10 +7,15 @@
 * create a database user called <user name> with password <key>
 ```
 sudo -u postgres psql -W
-CREATE USER <user name> with encrypted password <key>;
+CREATE USER <user name> with encrypted password '<key>';
 grant all privileges on database <database name> to <user name>;
 ```
 
+* OPTION: if <user name> already exist, pw can be changed this way
+```
+ALTER USER <user name> WITH ENCRYPTED PASSWORD '<key>';
+```
+
 * OPTION: gant user <user_name> access to database ```postgres``` so that this user is able to login without providing a database
 ```
 sudo -u postgres psql -W