chore: update readme about hashed password

README.md

@@ -252,16 +252,20 @@ DUFS supports the use of sha-512 hashed password.
 
 Create hashed password
 
-```
-$ mkpasswd -m sha-512 123456
+```sh
+$ openssl passwd -6 123456 # or `mkpasswd -m sha-512 123456`
 $6$tWMB51u6Kb2ui3wd$5gVHP92V9kZcMwQeKTjyTRgySsYJu471Jb1I6iHQ8iZ6s07GgCIO69KcPBRuwPE5tDq05xMAzye0NxVKuJdYs/
 ```
 
 Use hashed password
-```
+```sh
+dufs -a "admin:$(openssl passwd -6 123456)@/:rw"
+dufs -a "admin:$(mkpasswd -m sha-512 123456)@/:rw"
 dufs -a 'admin:$6$tWMB51u6Kb2ui3wd$5gVHP92V9kZcMwQeKTjyTRgySsYJu471Jb1I6iHQ8iZ6s07GgCIO69KcPBRuwPE5tDq05xMAzye0NxVKuJdYs/@/:rw'
 ```
 
+> The hashed password contains `$6`, which can expand to a variable in some shells, so you have to use **single quotes** to wrap it.
+
 Two important things for hashed passwords:
 
 1. Dufs only supports sha-512 hashed passwords, so ensure that the password string always starts with `$6$`.