message: doc: remove unimplemented `-t` arg from `split-pane`
  Fixes
diff: @@ -41,17 +41,13 @@ same window.
  #### `split-pane`
  -`split-pane [--target,-t target-pane] [-H]|[-V] [terminal_parameters]`
  +`split-pane [-H]|[-V] [terminal_parameters]`
  Creates a new pane in the currently focused tab by splitting the given pane
  vertically or horizontally.
  **Parameters**:
  -* `--target,-t target-pane`: Creates a new split in the given `target-pane`.
  - Each pane has a unique index (per-tab) which can be used to identify them.
  - These indicies are assigned in the order the panes were created. If omitted,
  - defaults to the index of the currently focused pane.
  * `-H`, `-V`: Used to indicate which direction to split the pane. `-V` is
  "vertically" (think `[|]`), and `-H` is "horizontally" (think `[-]`). If
  omitted, defaults to "auto", which splits the current pane in whatever the
  