Remove deprecated -p option to resize the window

[danielepintore]

With the latest version of tmux there was a problem with the -p option causing tmux to return -1 and therefore blocking the splitting. From the tmux documentation:

The -l option specifies the size of the new pane in lines (for vertical split) or in columns (for horizontal split); size may be followed by ‘%’ to specify a percentage of the available space.

We can use the -l option to do both percentage and line resizing