docs: Fix links to Pango website (#4217)

Author: ragibson

docs/source/guides/using_text.rst

@@ -50,7 +50,7 @@ For example:
             )
             self.add(text)
 
-.. _Pango library: https://pango.gnome.org
+.. _Pango library: https://pango.org
 
 Working with :class:`~.Text`
 ============================

manim/mobject/text/text_mobject.py

@@ -301,7 +301,7 @@ def _change_alignment_for_a_line(self, alignment: str, line_no: int) -> None:
 
 
 class Text(SVGMobject):
-    r"""Display (non-LaTeX) text rendered using `Pango <https://pango.gnome.org/>`_.
+    r"""Display (non-LaTeX) text rendered using `Pango <https://pango.org/>`_.
 
     Text objects behave like a :class:`.VGroup`-like iterable of all characters
     in the given text. In particular, slicing is possible.
@@ -864,7 +864,7 @@ def init_colors(self, propagate_colors=True):
 
 
 class MarkupText(SVGMobject):
-    r"""Display (non-LaTeX) text rendered using `Pango <https://pango.gnome.org/>`_.
+    r"""Display (non-LaTeX) text rendered using `Pango <https://pango.org/>`_.
 
     Text objects behave like a :class:`.VGroup`-like iterable of all characters
     in the given text. In particular, slicing is possible.