You are able to see this document because of two things

the .htaccess file in this directory

AuthUserFile /usr/local/lib/htpasswd/passwd_ex2
AuthGroupFile /usr/local/lib/htpasswd/group_ex2
AuthName ExampleByGroup
AuthType Basic


<Limit GET>
require group html-users
</Limit>
  

the group file (kept elsewhere)

html-users: html sgml
  

and the password file (kept elsewhere)

This was generated by using

      htpasswd -c /usr/local/lib/htpasswd/passwd_ex2 html
      htpasswd /usr/local/lib/htpasswd/passwd_ex2 sgml