update documentation for PromptInputLine

This adds prompt argument information to the documentation

docs/config/lua/keyassignment/PromptInputLine.md

@@ -8,10 +8,12 @@ from the user.
 When the user enters the line, emits an event that allows you to act
 upon the input.
 
-`PromptInputLine` accepts two fields:
+`PromptInputLine` accepts three fields:
 
 * `description` - the text to show at the top of the display area. You may
   embed escape sequences and/or use [wezterm.format](../wezterm/format.md).
+* `prompt` - the text to show as the prompt. You may embed escape sequences
+  and/or use [wezterm.format](../wezterm/format.md).  Defaults to: `"> "`
 * `action` - and event callback registerd via `wezterm.action_callback`.  The
   callback's function signature is `(window, pane, line)` where `window` and
   `pane` are the [Window](../window/index.md) and [Pane](../pane/index.md)