[tmux/en] adding info about copy mode

[Kyle-Mendes]

• I solemnly swear that this is all original content of which I am the original author
• Pull request title is prepended with [language/lang-code]
• Pull request touches only one file (or a set of logically related files with similar changes made)
• Content changes are aimed at intermediate to experienced programmers (this is a poor format for explaining fundamental programming concepts)
• If you've changed any part of the YAML Frontmatter, make sure it is formatted according to CONTRIBUTING.md
  • Yes, I have double-checked quotes and field names!

Just adding a simple snippet about copy mode. I use it a lot but used to forget what the key command was!

[divayprakash]

Thank you for the contribution!