Update ui_main_menu.rst (#922)

* Update ui_main_menu.rst

Updated link to point to the resolution managing / stretching page, was pointing nowhere.

* Update ui_main_menu.rst

Edited final link to point to the doc page, in a portable way thx to skyace65.

learning/step_by_step/ui_main_menu.rst

@@ -294,5 +294,4 @@ You can change the scale mode in the project settings: click the Project menu ->
 Settings. In the window's left column, look for the Display category.
 Click on the Window sub-category. On the right side of the window,
 you'll find a Stretch section. The three settings, Mode, Aspect, and
-Shrink, control the screen size. For more information, see `Project
-Settings <#>`__.
+Shrink, control the screen size. For more information, see `:ref:'Project Settings<doc_multiple_resolutions>'`__.