Fix mismatched backticks in "2D navigation overview" (#11254)

* Fix mismatched backticks in "2D navigation overview"

* Add `button` role for "Bake NavigationPolygon" in "2D navigation overview"

Co-authored-by: A Thousand Ships <[email protected]>

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Co-authored-by: A Thousand Ships <[email protected]>

Author: Rageking8

tutorials/navigation/navigation_introduction_2d.rst

@@ -100,7 +100,7 @@ It uses the NavigationServer2D and a NavigationAgent2D for path movement.
    .. image:: img/nav_2d_min_setup_step1.png
 
 #. Define the movable navigation area with the NavigationPolygon draw tool. Then click
-   the `Bake NavigationPolygon`` button on the toolbar.
+   the :button:`Bake NavigationPolygon` button on the toolbar.
 
    .. image:: img/nav_2d_min_setup_step2.png