Fix base64 example to use universal option

What does this MR do?

Fixes doc to use base64 --decode

Below all confirm, we can use --decode

• https://linux.die.net/man/1/base64
• https://www.freebsd.org/cgi/man.cgi?query=base64&apropos=0&sektion=0&manpath=FreeBSD+12.0-RELEASE+and+Ports&arch=default&format=html
• https://www.unix.com/man-page/osx/1/base64/

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/55218

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer