grub2-mkpasswd-pbkdf2: add page

[ankushitguy]

grub2-mkpasswd-pbkdf2 added

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[tldr-bot]

The build for this PR failed with the following error(s):

pages/linux/grub2-mkpasswd-pbkdf2.md:6: TLDR005 Example descriptions should end in a colon with no trailing characters

Please fix the error(s) and push again.

[ankushitguy]

changes done

[ankushitguy]

changes done

[spageektti]

Hi. Have you seen the style guide and contributing guidelines?