Merge branch 'doc/fix-due-date-in-slash-commands' into 'master'

Fix due date example in slash commands documentation

## What does this MR do?

Fixes due data example in documentation for slash commands.

## Screenshots

Before:

![due_date_slash_commands_before](/uploads/63484bfcad6605e7caf2a98f39ae65d0/due_date_slash_commands_before.png)

After:

![slash_commands_brackets](/uploads/5a75563eb797b4d694c6e4eca946e17a/slash_commands_brackets.png)

See merge request !5931