Update instructions for using OSC52 with tmux

[EliasA5]

• PR Description
  Update the instructions on using copyToClipboardCmd config to copy using OSC52 with tmux

Added -w 0 flag to base64 command because it wraps lines that are longer than 76 by default, that fails to copy on some terminal emulators.

We compare $TERM to ^(screen|tmux) because both of them are valid as per the manual:

default-terminal terminal
               Set the default terminal for new windows created in this
               session - the default value of the TERM environment
               variable.  For to work correctly, this must be set to
               ‘screen’, ‘tmux’ or a derivative of them.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...) -- not relevant
• Code has been formatted (see here) -- not relevant
• Tests have been added/updated (see here for the integration test guide) -- not relevant
• Text is internationalised (see here) -- not relevant
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc