Update tmux.html.markdown

Corrected page structure/style to conform to other tutorials
```
Line  18: Removed "tmux" as it has other purposes as well as quickly starting a blank session that will not be covered
Line  45: Added Meta/Alt explanation 
Line 126: Moved 'bind-key C-a send-prefix" to the group that requires it
Line 184: Added comment 
Line 186: Added comment pointing to resource required (if enabled)

- Removed 'set-option -g status-justify centre'

```