onSetActive doc: move it

Cosmetic change: move onSetActive description to keep it together with
other callbacks that are associated with bufpane, not with buffer.

Author: dmaluka

runtime/help/plugins.md

@@ -51,14 +51,14 @@ which micro defines:
 
 * `postinit()`: initialization function called after `init()`.
 
-* `onSetActive(bufpane)`: runs when changing the currently active bufpane.
-
 * `onBufferOpen(buf)`: runs when a buffer is opened. The input contains
    the buffer object.
 
 * `onBufPaneOpen(bufpane)`: runs when a bufpane is opened. The input
    contains the bufpane object.
 
+* `onSetActive(bufpane)`: runs when changing the currently active bufpane.
+
 * `onAction(bufpane)`: runs when `Action` is triggered by the user, where
    `Action` is a bindable action (see `> help keybindings`). A bufpane
    is passed as input and the function should return a boolean defining