drm/panel: Fix todo indentation for panel prepared/enabled cleanup

In commit d2aacaf0 ("drm/panel: Check for already prepared/enabled
in drm_panel") the formatting for a code block was not quite
right. This caused an error when building htmldocs:

  Documentation/gpu/todo.rst:469: ERROR: Unexpected indentation.

Fix the error by using the proper syntax for a code block.

Fixes: d2aacaf0 ("drm/panel: Check for already prepared/enabled in drm_panel")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/r/20230802141724.0edce253@canb.auug.org.auSigned-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230802074727.2.Iaeb7b0f7951aee6b8c090364bbc87b1ae198a857@changeid

Documentation/gpu/todo.rst

@@ -465,7 +465,8 @@ Clean up checks for already prepared/enabled in panels
 
 In a whole pile of panel drivers, we have code to make the
 prepare/unprepare/enable/disable callbacks behave as no-ops if they've already
-been called. To get some idea of the duplicated code, try:
+been called. To get some idea of the duplicated code, try::
+
   git grep 'if.*>prepared' -- drivers/gpu/drm/panel
   git grep 'if.*>enabled' -- drivers/gpu/drm/panel