Updating of old URL (Pango Markup formatting page) (#2704)

The old URL https://developer.gnome.org/pango/stable/pango-Markup.html redirects to https://docs.gtk.org/Pango/ which is a generic page, not specific to the Pango Markup formatting. I replace it with the new address of the documentation: https://docs.gtk.org/Pango/pango_markup.html

Author: quark67

manim/mobject/text/text_mobject.py

@@ -888,7 +888,7 @@ def construct(self):
 
     You can find more information about Pango markup formatting at the
     corresponding documentation page:
-    `Pango Markup <https://developer.gnome.org/pango/stable/pango-Markup.html>`_.
+    `Pango Markup <https://docs.gtk.org/Pango/pango_markup.html>`_.
     Please be aware that not all features are supported by this class and that
     the ``<gradient>`` tag mentioned above is not supported by Pango.