Step by step fixes (#1261)

* Fix image description

* Fix MarginContainer tutorial

Fixed the values for the right and bottom margins of margin container. They should be negative.
Moved the Layout -> Full Rect step to before setting the margins because selecting a layout removes the margins you've already described.

getting_started/step_by_step/ui_introduction_to_the_ui_system.rst

@@ -262,7 +262,7 @@ edges of its parent, or until it's blocked by another UI node.
 
 .. figure:: img/textureframe_in_box_container_expand.png
 
-   The same example as bove, but the left node has the "Expand" size
+   The same example as above, but the center node has the "Expand" size
    flag
 
 You'll need some practice to understand the size tags, as their effect

getting_started/step_by_step/ui_main_menu.rst

@@ -128,17 +128,16 @@ should be the outermost container or element. In this case it's a
 most interfaces, as you often need padding around the UI. Press
 ``Meta+S`` to save the scene to the disk. Name it *MainMenu*.
 
-Select the ``MarginContainer`` again, and head to the inspector to
-define the margins' size. Scroll down the ``Control`` class, to the
-``Custom Constants`` section. Unfold it. Set the margins as such:
+Select the ``MarginContainer`` again. We want the container to fit the 
+window. In the Viewport, open the ``Layout`` menu and select the last 
+option, ``Full Rect``. Head to the inspector to define the margins' size.
+Scroll down the ``Control`` class, to the ``Custom Constants`` section. 
+Unfold it. Set the margins as such:
 
--  Margin Right: *120*
+-  Margin Right: *-120*
 -  Margin Top: *80*
 -  Margin Left: *120*
--  Margin Bottom: *80*
-
-We want the container to fit the window. In the Viewport, open the
-``Layout`` menu and select the last option, ``Full Rect``.
+-  Margin Bottom: *-80*
 
 Add the UI sprites
 ~~~~~~~~~~~~~~~~~~